DEV Community: TAKUMI SUGATA

Making Tokyo and Osaka the Same Floor — Understanding VLAN, VXLAN, and VNI

TAKUMI SUGATA — Tue, 02 Jun 2026 01:44:14 +0000

Introduction

While studying for CompTIA Network+, I couldn't grasp what VNI is.
I could only memorize that " VNI is the number used to identify a VXLAN network", but I couldn't totally understand what it is and what it differ from VLAN.

Once I matched familiar example with it, everything fell into place, so I decided to write it down.

The difference between VLAN and VXLAN

First of all, we need to understand the difference between them to comprehend VNI.

What is VLAN

VLAN = Virtual LAN

A mechanism for dividing physical network into virtual segments within the same switch or  building.

┌────────────────────────────┐
│  A switch                  │
│  VLAN10 │ VLAN20 │ VLAN30  │
└────────────────────────────┘

Physical constraint：
　→ The same switch or until the adjacent switch
　→ It cannot reach long distance
　→ Max : 4094

VLAN limitation

In a cloud environment, it is essential to divide network for each tenant ( enterprise )
Thousand or ten thousand networks are needed
　↓
VLAN's maximum of 4094 is far from sufficient for this scale

What is VXLAN

VXLAN = Virtual Extensible LAN

A mechanism that extends VLAN to create networks beyond physical boundaries.

Datacenter A            Datacenter B
┌─────────┐             ┌─────────┐
│ Server  │─── VXLAN ───│ Server  │
└─────────┘    Tunnel   └─────────┘

VLAN：
　→ No tunnel
　→ Segments are divided using physical switch ports

VXLAN：
　→ UDP Tunnel（ Port 4789）
　→ Layer 2 Frame is transferred on Layer 3 network
　→ It can overcome the physical constraint

Compare VLAN with VXLAN

Category	VLAN	VXLAN
Purpose	Divide physical NW	Create NW beyond place
Identifier	VLAN ID（12bit）	VNI（24bit）
Maximum	4094	16million
Range	Inside building	Regardless of the place
Transfer	Layer 2	UDP（ On Layer 3）
Use	Small or medium-scale environment	Large-scale cloud environment

Understanding with familiar example

VLAN：
　→ Segment rooms for each floor inside a building 
　→ It can just us inside the area

VXLAN：
　→ It can use every office (Tokyo, Osaka, Fukuoka) virtually as the same floor
　→ It can surpass physical distance

What is VNI

VXLAN Network Identifier

In a nutshell, ** it is the number used to identify a virtual network created by VXLAN **.

VLAN ID = VLAN identification number（12bit・max 4094）
VNI     = VXLAN identification number（24bit・max 16million）

VNI is an extended version of VLAN ID
　→ VLAN ID is 12bit
　→ VNI is 24bit
　→ This allows approximately 4000 times more identifiers

Understanding with postcode

VLAN ID：
　→ Apartment room number （Max 4094 rooms）
　→ Lack for large-scale environment

VNI：
　→ Country's postcode（ Max 16 million）
　→ It can accommodate any scale of environment

The mechanism of VXLAN

VXLAN uses a UDP tunnel to transfer Layer 2 frames over Layer 3 network

┌───────────────────────────────────────────────────────────────┐
│         VXLAN Packet                                          │                                                                                   │                                                               │
│  ┌─────────────────────────────┐                              │                                      │  │                             │                              │
│  │    Original Layer 2 frames  │                              │                                       │  │                             │                              │
│  └─────────────────────────────┘                              │                                
│ VNI（ Identifier that specifies                               │
│                 which virtual network the packet belongs to） │
└───────────────────────────────────────────────────────────────┘
　　　　　　↓
　　Transfer on UDP port 4789

Summary

VLAN ：
The mechanism to divide network inside building
　→ Max 4094・12bit

VXLAN ：
The mechanism to create virtual network beyond building
　→ Max 16million・UDP tunnel

VNI ：
An identifying number used to distinguish virtual networks created by VXLAN
　→ 24bit・Max 16 million
　→ Extended version of VLAN ID

The difference between VLAN and VXLAN：
VLAN  → Inside the same building・ Max 4094
VXLAN → Regardless of the building・ Max 16 million

Conclusion

I used to struggle with memorizing that VNI is the identification number for VXLAN.
Once I understood the structure behind each term, I could naturally grasp why VNI is essential.

IDS Is a Fire Alarm, IPS Is a Sprinkler — Understanding Snort from the Ground Up

TAKUMI SUGATA — Sun, 31 May 2026 02:27:41 +0000

Introduction

While studying for CompTIA Network+, I couldn't understand a relationship between Snort and IDS. I could memorize that Snort is an opened source tool, but I couldn't totally comprehend the link between Snort and IDS and why IDS is needed.
Once I grasped the structure of both systems, everything clicked, so I decided to write it down.

What is IDS

IDS stands for Intrusion Detection System
In a nutshell, it is a system that ** detects suspicious access to a network and alerts administrators**

Important point

IDS = Detect and Alert
　→ Do not block automatically

The system for blocking：
IPS（　Intrusion Prevention System）

The difference between IDS and IPS

IDS：
Detect suspicious accesses
　↓
Alert to a system manager 
　↓
The system manager judges it and block it manually

IPS：
Detect suspicious accesses
　↓
Automatically block
　↓
Notify it to a system manager

Understanding with a fire alarm

IDS = A fire alarm
　→ Alert in detecting fire
　→ It does not automatically extinguishing fire
　→ Human do on behalf of it

IPS = Sprinkler
　→ Automatically firefighting in detecting fire
　→ It can deal with it before human does

What is Snort

Snort is an open-source tool for IDS/IPS

Developed by： Cisco
License： Open-source（ free ）
Support OS： Linux・ Windows・ macOS

Main features：
・Real-time monitoring for network traffic
・Detecting suspicious packets
・Working based on rules
・Notifying system administrators and logging reports

Operating mode by Snort

Snort has three operating modes

1. Sniffer Mode

Just show real-time packets
　→ Do not report
　→ It is used when you check network status

2. Packet Logger Mode

Report packet information to a disk
　→ It makes you analyze the information later
　→ It is used in troubleshooting

3. Network IDS Mode (IDS Mode)

After analyzing packets, verify it with rules
　→ Detect suspicious packets and notify it
　→ The most commonly used mode
　→ The most frequently tested in exams

What is rule based

Snort detects suspicious packets based on rules, also known as signatures

Example of rule：

"A large amount of access attempts to Port 22"
　→ Possible unauthorized access to SSH
　→ Notification for managers

"Access from a known attacker's IP address"
　→ Possible suspicious access detected
　→ Notification for managers

The mechanism of rule based

Network traffic
　↓
Snort monitors packets
　↓
Verify it with rules
　↓
Match → Detect the access as suspicious and notify it
Mismatch → Pass through as an normal access

Pros and Cons

Pros：
・Free due to being open-source
・There is a large community
・It can customize rules by yourself
・It is used all over the world

Cons：
・Complex settings
・Automatic blocking requires additional IPS configuration
・False Positive may occur

What is False Positive

A false Positive occurs when normal access is incorrectly detected as suspicious

Example：
It incorrectly flags legitimate employee access as an attack

Solution：
Tuning rules properly

Relationship with OSI reference model

Layer 7  Application  ← Detect attacks at application layer as well
Layer 6  Presentation │
Layer 5  Session      │
Layer 4  Transport    ← Detect port scan etc...
Layer 3  Network      ← Detect IP address based attacks 
Layer 2  Data Link    │
Layer 1  Physical     │
─────────────────────────────
Snort primarily operates across Layer 3 go Layer 7

Summary

What is IDS：
The mechanism to detect suspicious access to network and notify it

The difference between IDS and IPS：
IDS → Detect and notify（ Manual ）
IPS → Detect and automatically block

What is Snort：
IDS/IPS tool offered by opened source

Operating mode：
1. Sniffer Mode     → Just show
2. Packet Logger    → Record
3. Network IDS Mode → Detect and notify（ The most used mode ）

Features：
・Work based on rules
・Opend source and free
・Real-timed monitoring
・Be careful for False Positive

Conclusion

I used to struggle with understanding Snort because I was only trying to memorize it.
Once I understood the differences outlined below, I could answer related questions naturally and grasp why Snort is needed.

IDS  = Detect・ Notify（ A fire alarm ）
IPS  = Detect・ Automatically Block（ Sprinkler ）

Snort = IDS/IPS tool based on rules

Understanding TPC with IEEE802.11h

TAKUMI SUGATA — Sun, 24 May 2026 23:08:54 +0000

Introduction

While studying for CompTIA Network+, I couldn't imagine how TPC worked. I could memorize that TPC stands for Transmit Power Control, but I couldn't comprehend why it was needed or how it actually worked.
Once I grasped TCP, everything fell into place. So, I decided to write it down.

What is TPC

TPC stands for Transmit Power Control
In a nutshell, it is a mechanism that automatically adjusts transmission power for Wi-Fi.

Why TPC is needed

If transmission power is too strong：
　→ Radio waves spread further than necessary
　→ Interfere with adjacent access point
　→ Increase power consumption in vain
　→ Radio wave end up reaching to outside of building (Security Risk)

If transmission power is too weak：
　→ Radio wave cannot reach to an expected range
　→ Connection is unstable

Solution

Automatic adjust depends on the situation is needed
　→Deliver radio waves only where needed 
　→ Decrease interference
　→ Save power consumption

Familiar example

Volume control for microphone in meeting

When you talk to person close to you
　→Small volume is enough（ Decrease electric power）

When you talk to person far from you
　→Big volume is required（ Increase electric power）

TPC automatically adjusts the volume for you

How TPC control

TPC is not for detecting devices but for adjusting electric power after measuring the strength of radio wave and feeding back it.

1. Devices connect to an access point
　↓
2. Devices measure the strength of the radio waves they receive
　↓
3. Devices report the strength to the access point
　↓
4. The access point adjust transmission power depends on the information
　↓
5. The access point transmits at only power level required

Pros for TPC

1. Reduce interference
　→ Do not spread radio wave more than it is needed
　→ Decrease interference with an adjacent access point

2. Save power consumption
　→ Use only electric power is needed
　→ Extend battery life

3. Enhance security
　→ Ensure range radio wave reach 
　→ Prevent radio wave leaking outside a building

4. Improve overall network efficiency
　→ Multiple access points work with adequate electric power
　→ Improve performance

Relationship with IEEE 802.11h

What is IEEE 802.11h

 IEEE 802.11h  is a Wi-Fi standard that formally defines TPC and DFS as standard specifications

Why 802.11h is created

Background：
5GHz band has been originally used in military, weather radar, satellite communication

Problem：
Once Wi-Fi began using the same frequency band, interference with existing radar systems became a concern.

Solution：
Standardize TPC and DFS with 802.11h
　→ Don’t output radio wave more than needed ( TPC )
　→ Avoid frequency interfering with radar ( DFS )

The role of TPC and DFS

TPC（ Transmit Power Control）
　→ Adjust transmission power
　→ Decrease interference and save power consuming

DFS（ Dynamic Frequency Selection）
　→ Automatically switch to other frequency when it detects radar
　→ Avoid interference with radar systems

The relationship between TPC and 802.11h

TPC    = Mechanism (concept)  for automatically adjusting transmission power
802.11h = Defining TPC and DFS as standard

Although TPC existed as a concept before 802.11h, it was formally defined as an official standard by 802.11h

The region 802.11h is needed

It is a duty in mainly Europe

EU regulation：
Wi-Fi devices that use the 5GHz band are required to comply with 802.11h（TPC・DFS）

In fact, it is also common in Japan and North America

The relationship with OSI reference model

Layer 7  Application  │
Layer 6  Presentation │
Layer 5  Session      │  The layers TPC is not applicable
Layer 4  Transport    │
Layer 3  Network      │
Layer 2  Data Link    │
─────────────────────────────
Layer 1  Physical     ← The  layer TPC works
─────────────────────────────

TCP operates at Layer 1 to control the physical strength of radio waves

Summary

TPC ：The method to automatically adjust transmission power of Wi-Fi

How it works： Devices measure the strength of received raido waves and report back to the access points
　→ Access Point automatically adjust transmission power

Pros：
・Reduce interference
・Save power consumption
・Improve security
・Improve efficiency in whole network

Relationship with 802.11h：
TPC = Concept for automatic adjusting transmission power
802.11h = Define TPC and DFS as standard
　→ It is created for prevent interference from radar in 5GHz band

OSI Layer： Physical Layer（ Layer 1）

Conclusion

I used to struggle with even memorizing the term itself, but everything clicked once I understood the relationship between TPC and 802.11h.

TPC = The method to automatically adjust the strength of radio wave depending on the situation 802.11h = The definition of TPC as standard

Crosstalk is Just Like Music Leaking from Someone's Earphones on a Crowded Train.

TAKUMI SUGATA — Wed, 20 May 2026 22:52:45 +0000

Introduction

While studying for CompTIA Network+, I couldn't visualize what Crosstalk actually meant. I could memorize that it refers to interference from adjacent copper cable. But, I didn't understand why the interference occurs and why the finer twist is faster.

Once I understood from the structure, everything fell into place.

What is Crosstalk

To receive electric interference from copper cable.

An electromagnetic field occurs around a copper cable when electric current flows 
　↓
The electromagnetic field affects the other copper cables 
　↓
Unintended signals ride on the copper cables
　→ This is Crosstalk

Familiar example

In a crowded train, you may have experienced music leaking from someone else's earphones. 
That leaked sound mixes with the music you are listening.
　→ This is Crosstalk image

The problems Crosstalk cause

・Decline in communication speed
・Data corruption
・Unstable connection
・Rise in error rate

2 kinds of Crosstalk

There are 2 kinds of Crosstalk depend on occur place

1. Internal Crosstalk (Inside cable)

8 Copper cables interfere each other inside the same LAN cable 

┌─────────────────┐
│ ○ ○ ○ ○ ○ ○ ○ ○ │ ← 8 cables inside LAN cable 
└─────────────────┘
　　↑
This copper cable interfere each other in electromagnetic field

Feature：
・The most common Crosstalk
・Cannot be physically separated since the interference occurs inside the cable itself. 
・Twisted pair and shielding are the solusions

2. External Crosstalk / Alien Crosstalk (Between cables)

Other cables adjacent to a cable interfere each other 

Cable A ──────────────
Cable B ──────────────  ← Run side by side
　↑
These two cables interfere mutually

Feature：
・It tends to occur when cables are bundled or run side by side
・The interference will be greater when it run next to a power cable
・Physically separating the cables is the solution

Why more twisting reduce interference

This is the core concept of this article

The mechanism of Twisted pair

When two copper cables are twisted ：

Copper cable A → Generates a clockwise electromagnetic field 
Copper cable B → Generates a counterclockwise electromagnetic field
    ↓
Each electromagnetic field cancels each other out
　↓
Leakage to outside decreases

The relationships between rough twisting and interference

Coarse twisting：
　→ Cancellation is weak 
　→ More leakage
　→ Huge interference（Frequent Crosstalk）

Fine twisting：
　→ Cancellation is strong
　→ Less leakage
　→ Minimal interfeernce（Less Crosstalk）

More Twists → Less Leakage → High Speed

More Twists
　↓
Cancellation in electromagnetic field is stronger
　↓
Less Leakage
　↓
Signals are stable
　↓
High Speed

** This is the reason why higher CAT standard enable faster communication speeds **

Solution for Crosstalk

Solutions differ from each kind

Solution for Internal Crosstalk

** 1. Twisted pair **

Every 2 copper cables are twisted together
　→ Cancellation in electromagnetic field
　→ Decline in interference

The more twisting, the greater the effect
　→ Higher CAT standard is more twisting

*2. Shield *

Over CAT6a：Whole cables are covered with shield
CAT7    ：Each pair are covered with individual shield

It can prevent leakage from inside electromagnetic field

Solution for External Crosstalk

** 1. Physical separation **

Do not bundle each LAN cable
Do not run next to power cable 
　→ Physical separation can reduce interference

*2 . Shield *

The cable covered with shield are used
　→ It blocks interference from outside sources

Solution summary table

Solution	Internal Crosstalk	External Crosstalk
Twisted pair	✓ Valid	△ Less valid
Shield	✓ Valid	✓ Valid
Physical separation	✗ Invalid	✓ Valid

Relationship with CAT standard

CAT5  → Coarse twisting
　→ More leakage・ More interference → Until 100Mbps

CAT5e → More twisting
　→ Less leakage・ Less interference → 1Gbps

CAT6  → More twisting + spine added to the center
　→ 10Gbps（Until 55m）

CAT6a → Add whole shield
　→ Prevent from interference from outside  → 10Gbps（100m）

CAT7  → Add individual shield to each pair
　→ No Internal Crosstalk → 10Gbps（100m）

Summary

Crosstalk ：The phenomenon of receiving electrical interference from copper cables

2 kinds of Crosstalk：
Internal Crosstalk
　→ Interference caused by copper cable in the same cable 
　→ Cannot separate physically
　→ Twisted pair・ Shield are the solution

External Crosstalk （Alien Crosstalk）
　→ Interference caused by adjacent cables
　→ Can separate physically
　→ Shield is also the solution

Core：
More Twists → Less Leakage → Less Crosstalk → High Speed
　→ This is the reason why higher CAT standards rise in speed

Conclusion

I used to struggle with memorizing what Crosstalk was.
Once I understood the flow below, everything clicked naturally.

Internal Crosstalk  = Interference inside cable
　→ Solution : Twisted pair・ Shield

External Crosstalk  = Interference between cables
　→ Solution : Physical separation・ Shield

More Twists → Less Leakage → High Speed

Even Without Typing a Command — Understanding What show vlan Actually Shows

TAKUMI SUGATA — Mon, 18 May 2026 22:42:34 +0000

Introduction

While studying for CompTIA Network+, I couldn't totally understand what "show VLAN command" shows. I have never typed command on my computer and seen the output.

I understood what VLAN was and what the command actually shows, everything clicked. So, I decided to write it down.

What is VLAN

At first, before understanding Show VLAN command, we need to know VLAN.

VLAN = Virtual Local Area Network
It is a mechanism that divides a physical network into virtual segments.

Why VLAN is needed

Without VLAN：
　→ Everyone is in the same network
　→ All data are shared across the entire organization regardless of department
　→ Problem for security and performance

With VLAN：
　→ Be able to divide network depend on department
　→ Only sales staff can access data managed by the sales department
　→ Ensure security

Familiar example

Without VLAN：
　→ Everyone work in one room
　→ Every conversation can be heard to everyone

With VLAN：
　→ Each department has its own private room
　→ Conversation cannot be overheard by other departments

Configuration diagram

【Without VLAN】

Switch
　├── PC（Sales department）
　├── PC（Development department）  ← Everyone in the same network
　└── PC（Management department）

【With VLAN】

Switch
　├── VLAN 10（Sales department）── PC・PC
　├── VLAN 20（Development department）── PC・PC  ← Divide network into each department
　└── VLAN 30（Management department）── PC・PC

What is show VLAN command

It is the *command to verify VLAN information on a Cisco switch *.

Caution：
The show vlan command is exclusive to switches running Cisco IOS 

Other vendors use other command：
　Juniper  → show vlans
　HP/Aruba → show vlans
　Dell     → show vlan

Kinds of the command

There are three main command on this.

1. show vlan

Show All VLAN information in detail

2. show vlan brief

Show VLAN overview simply
　→ The most used command
　→ Frequently appears in the exam

3. show vlan id [Number]

Just show specific VLAN

例：
show vlan id 10
　→ Show only VLAN 10 information

Output images

Output example for show vlan brief.

VLAN Name                Status    Ports
---- -------------------- --------- ------
1    default              active    Gi0/1, Gi0/2
10   Sales                active    Gi0/3, Gi0/4
20   Engineering          active    Gi0/5, Gi0/6
30   Management           active    Gi0/7
1002 fddi-default         act/unsup
1003 token-ring-default   act/unsup

Meanings of each row

VLAN：
　→ VLAN ID（ Identification number）
　→ Available range 1〜4094

Name：
　→ VLAN name

Status：
　→ active    = Work normally
　→ act/unsup = Not supported

Ports：
　→ Ports assigned to the VLAN
　→ Gi = GigabitEthernet

About Default VLAN (VLAN 1)

VLAN 1 is a special case

・All ports belong to VLAN 1 as default
・Cannot be deleted
・Cannot be changed from name "default"

Caution for security

It is dangerous to use VLAN 1 itself
　↓
Reason：
Misconfiguration can cause unintended traffic across the network because all ports belong to VLAN 1 as default.

Measure：
It is common to move to other VLAN in production environment

When the command is used

Troubleshooting

Verifying whether VLAN setting are correctly configured .

Example：
PC cannot connect to network
　↓
Type "show vlan brief" and confirm the output
　↓
Discover problem that this port was not assigned to VLAN

Setting confirmation

After creating a new VLAN, run this command to verify the settings were applied correctly.

Example：
Confirm whether settings success correctly by type this command after new VLANN is created

Adding new devices

Setting after confirming "Which vlan we have to add to"

Example：
We want to add a new PC to the sales department network
　↓
Confirming VLAN ID for sales department by typing "show vlan brief" 
    ↓
Assigning the port to the VLAN

Command comparison in each vendor

Vendor	Command
Cisco	`show vlan` / `show vlan brief`
Juniper	`show vlans`
HP / Aruba	`show vlans`
Dell	`show vlan`

Summary

VLAN ：
To divide physical network into segment virtually
It can ensure security by network segmentation 

Show VLAN Command：
The command to confirm VLAN information on Cisco IOS Switch 

Main command：
show vlan       → Show detailed information
show vlan brief → Show overview（ Most frequently used）
show vlan id XX → Show specific VLAN information

Use case：
・Troubleshooting
・Setting confirmation
・Adding new devices to VLAN

Caution：
・Command dedicated to Cisco IOS
・All port belong to VLAN 1 as default
・Don't use VLAN 1 in production environment

Conclusion

At first, I had no idea what the show vlan command was actually displaying. Once I understood that VALN is a mechanism for dividing a network into virtual segments, the output - including the meaning of each column - fell into place naturally.

Measure? Select? - Understanding the difference between Network metrics and Routing metrics

TAKUMI SUGATA — Tue, 12 May 2026 23:40:37 +0000

Introduction

While studying for CompTIA Network+, I totally couldn't understand a difference between Network Metrics and Routing Metrics. Both terms have "Metrics", which confused me.
Once I understood why each one existed, everything fell into place. So I decided to write it down.

Difference between two Metrics

As a premise, two Metrics are used for different purposes.

Network Metrics：
　→ Indicator for measuring performance of network
　→ System managers use it for surveillance or troubleshooting

Routing Metrics：
　→ Value for selecting the best path
　→ Routers use it automatically

Network Metrics

Indicator for measuring performance of network
In a CompTIA Network+, 4 metrics are particularly important.

1. Bandwidth

Keyword：Width of road
Meaning：Maximum amout of data that can be sent at one time
Unit：bps・Mbps・Gbps
Good condition：Higher is better

Wide road → It enables many vehicles to drive at the same time
Large Bandwidth → It enebles large amounts of data to be sent at once

2. Latency

Keyword：Period of time signal reaches
Meaning：the time it takes for data to travel from sender to receiver
Unit：ms（milliseconds）
Good condition：Lower is better (Under 20ms is preferable)

Example：
Online video games → 1ms（5G）
4G communication → 30〜50ms
Connection to servers overseas → over 100ms

3. Jitter

Keyword：Variation in delay
Meaning：Latency becomes inconsistent rather than remaining constant
Unit：ms
Good condition：Lower is better

Latency is stable（Low Jitter）：
　→ 10ms・10ms・10ms・10ms

Latency is unstable（High Jitter）：
　→ 10ms・50ms・5ms・80ms

In case of high Jitter：
・Video call and online video game are lagged
・Quality of voice through VoIP become worse

4. Packet Loss

Keyword：Data loss
Meaning：Portion shows that data don't reach to the destination 
Unit：%
Good condition：Lower is better (0% is preferable)

Calculation for Packet Loss
In case that 95 packets are reached even if 100 packets are served：
　→ Packet Loss = 5%

In case of high Packet Loss：
・Delay to load Web pages
・Disturb video call
・Fail to download files

Summary table

Metrics	Meaning	Unit	Good condition
Bandwidth	Maximum amount of data transfer	Mbps・Gbps	Higher is better
Latency	Delay time for signal	ms	Lower is better
Jitter	Variation in travel time	ms	Lower is better
Packet Loss	Data loss	%	Lower is better

Understanding with highway

Bandwidth   = Width of road（The number of lanes）
Latency     = Arrival time
Jitter      =Bariation of required time
Packet Loss = The proportion for car going missing on the way

Relationships between each metric

Even if bandwidth is bigger, it makes you feel slow due to high latency
Even if latency is lower, call will be low quality due to high jitter
Even if above all is good condition, it makes you feel slow for resending due to high packet loss

Routing Metrics

** It is a value used by routing protocol to prioritize available routes. It works as cost or score for specific route. **

In a nutshell, ** score for network route**.

Why is it needed

Problem：
When multiple routes exists to the same destination, it becomes difficult to determine which one is optimal.

Solution：
Attach value (metric) on each route
　→ Give priority to the route having lower value
　→ Router can select the best route automatically

Meaning of cost and score

cost：
　→ Lower is better
　→ Image of the cost for going through the way

score：
　→ Evaluation value
　→ It changes whether higher or lower is better depending on context

In CompTIA Network+ exam, it is enough to remember *cost ( lower is better) *.

Practical image

In case that there are three routes to the destination：
Route A：Metric = 10  ← Lowest value = Top priority
Route B：Metric = 50
Route C：Metric = 100 ← Highest value = Bottom priority

Router selects Route A which has the lowest metric value

Understanding with a car navigation

Route A：Highway（The distance is long but fast）→ cost 10
Route B：Public road（The distance is short but slow）  → cost 50
Route C：Mountain path（The distance is long and slow）  → cost 100

A car navigation ( Router ) select Route A which has the lowest cost

Kinds of routing metrics

Used value differs from each protocols.

Metrics	Meaning	Used protocol
Hop Count	The number of router that it go through	RIP
Bandwidth	Maximum speed on the route	OSPF
Latency	Delay time for overall route	EIGRP
Cost	Weigh set up by system managers	OSPF

Organize two metrics table

Item	Network Metrics	Routing Metrics
Purpose	Measure the performance	Select routes
Use case	Monitoring・Troubleshooting	Route selection by Router
Representative indicator	Bandwidth・Latency・Jitter・Packet Loss	Hop Count・Cost・Bandwidth・Latency
User	Network administrator	Router（auto）

Conclusion

Firstly, I couldn't distinguish the difference between each metric.
Once I understood the difference, the purpose behind each metric became clear.

Network Metrics  = Measure performance
Routing Metrics  = Select route

SSID vs ESSID — One Is a Name, the Other Is a Mechanism

TAKUMI SUGATA — Sun, 10 May 2026 08:34:17 +0000

Introduction

While studying for CompTIA Network+, I totally couldn't understand SSID and ESSID.
I could remember " SSID is the name of Wi-Fi ", but I couldn't grasp how it is different.
Once I learned about the structures, everything fell into place, so I decided to write it down.

What is SSID

SSID stands for Service Set Identifier

The name displayed on the smartphone for Wi-Fi settings like "○○-Wi-Fi","Coffee_Shop_Free"
　→All of these names are SSID

Why SSID is needed

Problem：
Radio waves are invisible to the human eye, so we have no way of knowing which Wi-Fi network to connect to.

Solution：
Name each Wi-Fi
　→ Users can identify and choose the network they want to connect to

Familiar example

Wi-Fi router at home
　→ The name of "My_Home_Wi-Fi"
　→ This is SSID

We can use Wi-Fi inside a range that a router reach 
The signal weakens as we move further away

Configuration diagram

Smartphone
　↓ Connect to "My_Home_Wi-Fi"
Access point (One unit)
　↓
Internet

What is ESSID

ESSID stands for Extended Service Set Identifier

In a nutshell, it is a mechanism that allows multiple access points to share the same name, enabling seamless connectivity.

E means Extended
This is an extended concept that expands SSID across multiple access points

Why ESSID is needed

Problem：
Single access point cannot cover wide range area 

Example： University campus・ Airport・ Large commercial facility
　→ Each building requires its own access point to provide adequate coverage
　→ Each time we move to other building, we have to re-connect to the Wi-Fi

Solution：
Multiple access points share the same name
　→ Wherever we move, our devices can connect to it automatically
　→ We don't have to re-connect to the Wi-Fi

Practical example

Overall university campus
　→ Building A, B, and C have several access points
　→ All access points have the same name of "University_Wi-Fi" 
　→ This is ESSID

Wherever we move, our devices keep connecting to the same Wi-Fi name
→ Need not re-connect

Configuration diagram

　　　Smartphone（ Using while moving ）
　　　　　　　　　↓
┌──────────────────────────────────────────────┐
│  "University_Wi-Fi"（ESSID）                 │
│  ┌───────────┐ ┌───────────┐  ┌───────────┐  │
│  │ AP-1　    │ │ AP-2      │  │ AP-3      │  │
│  │ Building A│ │ Building B│  │ Building C│  │
│  └───────────┘ └───────────┘  └───────────┘  │
└──────────────────────────────────────────────┘
　　　　　　　　　↓
　　　 　　   Internet 

Even if we connect our devices to any access point, we can use it as a same network

The difference between SSID and ESSID

This is the most confused point

SSID：
　→ The name itself
　→ Identifier named for the single access point
　→ Only valid for that single access point

ESSID：
　→ The name + the concept for integration
　→ The mechanism multiple access points share the same name 
　→ Works through the coordination of multiple access points behind the scenes

Understanding with convenience store

SSID：" Seven-Eleven ○○store"
　→ The unique name
　→ The name cannot use the other store for identification

ESSID：" Seven-Eleven" as a a brand name
　→ Every store offers the same service and products
　→ Each store ( access point ) integrate each other on the backend

Summary table

Item	SSID	ESSID
Official name	Service Set Identifier	Extended Service Set Identifier
Meaning	The name of a Wi-Fi network	A mechanism where multiple access points share the same name
Scope	Single access point	Multiple access points
Use	House ・Small scale	Office・ University・ Large scale facility
Reconnection	Every time we move to the other area	No need

Supplement in the modern era

Actual fact：
SSID  → Identifier for single access point
ESSID → Identifier bundled by multiple access points 

In the modern era：
In practice, SSID and ESSID are used interchangeably, as Wi-Fi settings typically display both simply as "SSID"

For the CompTIA Network+ exam, it is sufficient to understand ESSID as an extended concept that unifies multiple access points under a single network name.

Conclusion

I couldn't understand the difference between SSID and ESSID.
Once I understood why we don't need to reconnect every time we move around an airport or university campus, everything clicked.

Four network protocols (LACP, LLDP, L2TP, and LDAP) that are hard to memorize

TAKUMI SUGATA — Sat, 09 May 2026 02:59:26 +0000

Introduction

While studying for CompTIA Network+, many network protocols made me messed up.
It confused me a lot because of the similar names such as LACP, LLDP, L2TP, and LDAP.
I didn't understand each name and how they act in network.
Once I understood why each one existed, everything fell into place. So, I decided to write it down.

Overview table

Protocol	Purpose	Layer	Similar example
LACP	Bundle cables	Layer 2	1 Lane→4 Lanes
LLDP	Discover adjacent devices	Layer 2	Greet neighbor
L2TP	Create VPN tunnel	Layer 2	Sending a letter in an envelop
LDAP	Manage user authentication	Layer 7	One staff ID card opens all doors

LACP ( Link Aggregation Control Protocol )

Understanding in a nutshell

This protocol is a mechanism that bundles multiple cables into a single logical link.

Why it is needed

Problem：
Only one cable limits the speed (Maximum 1 Gbps) 
The connection will be stopped, if it breaks down.

Solution：
Bundle multiple cables
　→ Accelerate the speed
　→ Continue to connect, if a cable breaks down.

Familiar example

One lane road （A normal cable）
　→ Cause traffic jam, which slow the speed down
　→ Impossible to function by accident or construction work

Four lane roads（ Bundle four cables by LACP）
　→ Avoid the cause of congestion
　→ Possible to function by accident or construction work due to the presence of other lanes

Specific structure

Normal：
Switch ──── 1Gbps ──── Server
　　　　   　　↑
　 　   Only one cable

LACP：
　　　　　　　┌── 1Gbps ──┐
Switch ──────┼── 1Gbps ──┼───── Server
　　　　 　　 ├── 1Gbps ──┤
　　　　　　　└── 1Gbps ──┘
　　Total 4Gbps（Logically one cable)

2 main pros

1. Expand bandwidth
　→ Logically four times as fast as only one cable, if four cables are bundled.

2. Ensure redundancy
　→ The remaining cables enable maintain the connection even if one cable fails.
　→ Endure technical issues

LLDP ( Link Layer Discovery Protocol )

Understanding in a nutshell

** This protocol automatically discovers information about neighboring devices.**

Why it is needed

Problem：
In a large scale network, it is hard to grasp that where devices are connected

Solution：
Devices automatically tell adjacent devices the own information each other

Familiar example

When you move home
　→ You don't find the people who live next door
　→ Greeting makes you understand who he or she is

Without LLDP
　→ Manager have to confirm each device by hand 

With LLDP
　→ Each device automatically tell their information each other
　→ Managers can understand the entire network structure without leaving their desk

Information it can collect

・Name of the device
・Kind of the device ( Switch, Router etc. )
・Port number
・IP address
・Manufacturer

Use case

Troubleshooting：
　→ Instant check "Which device is connected to this port"

Network management：
　→ Connect to the tool that automatically create configuration diagram

Replace device：
　→ New device automatically introduces itself to adjacent devices

L2TP ( Layer 2 Tunneling Protocol )

Understanding in a nutshell

This protocol creates a virtual tunnel over the Internet to enable secure communication

Why it is needed

Problem：
Possible to leak important information to the Internet

Solution：
Create virtual tunnel and send data within the tunnel 
　→ Disabled to see the data from outside

Familiar example

Normal internet connection：
　→ Send with a postcard
　→ Anyone can read the content

L2TP tunnel：
　→ Send with an envelop
　→ No one can read the content
　→ Reach to the destination securely

Specific structure

Home PC
　↓
Internet（ Public line ）
　↓ L2TP tunnel（ Virtual dedicated line）
Office network

See from outside：
　→ Cannot see inside data
　→ Just like working at the office

Combination with IPsec

Only L2TP：
　→ Just create tunnel
　→ Without encryption

L2TP/IPsec：
　→ Tunnel + Encryption
　→ More secure VPN connection
　→ In practice, they are almost always used together

LDAP ( Lightweight Directory Access Protocol )

Understanding in a nutshell

This protocol is a mechanism that centrally manage user information and use them for authentication

Why it is needed

Problem：
A lot of login information are needed if there are many system
　→ Hard to manage multiple passwords

Solution：
Collect user information in one place and manage it centrally

Familiar example

Without LDAP：
　→ Separately manage several information such as Admission badge, PC password, mail password, system password
　→ Both managers and users are hard to manage them

With LDAP：
　→ Only one ID and password enables users to login all systems
　→ Admission badge makes us possible unlock all doors

Specific structure

PC（ Require login）
　↓
LDAP server（ Active Directory etc.）
　↓ " Has this user already been registered?"
　↓ " Has this user had which role?"
　↓
Authentication succeed → User can access the system
Authentication failed → User cannot access the system

What is Directory

Directory in LDAP is like address list for user information

Registered information：
・Name
・Mail address
・Department
・Password（ Hashed ）
・Role（ Common user or manager ）

Relationship with Active Directory

LDAP ：A protocol（ a set of communication rules ）
Active Directory： A Microsoft product that uses LDAP internally
LDAP is one of network protocols that Active Directory uses

Organize 4 protocols difference

LACP： Enhance physical connection
　→ Speed up and enforce redundancy by bundling cables

LLDP： Automatically identify network topology
　→ Automatically research what the adjacent device is

L2TP：Securely connect with remote area
　→ Create virtual tunnel in the Internet

LDAP： Manage users
　→ Centrally manage user authentication

Relationship with OSI reference model

Layer 7  Application  ← Where LDAP operates
Layer 6  Presentation │
Layer 5  Session      │
Layer 4  Transport    │
Layer 3  Network      │
─────────────────────────────
Layer 2  Data Link    ← Where LACP・LLDP・L2TP operate
Layer 1  Physical     │
─────────────────────────────

Summary ( repost )

Protocol	Purpose	Layer	Similar example
LACP	Bundle cables	Layer 2	1 Lane→4 Lanes
LLDP	Discover adjacent devices	Layer 2	Greet neighbor
L2TP	Create VPN tunnel	Layer 2	Sent letter in an envelop
LDAP	Management user authentication	Layer 7	Open all doors with an staff identity card

Conclusion

In this article, I covered four protocols that are easy to confuse due to their similar names. Once I understood the purpose behind each one, everything clicked. I hope this helps anyone else who is struggling with the same confusion.

I Finally Understood Routers and Switches by Connecting Them to Real-Life Examples

TAKUMI SUGATA — Thu, 07 May 2026 23:11:41 +0000

Introduction

While studying for CompTIA Network+, I couldn't totally grasp the difference between routers and switches. Even when I memorized their functions, the knowledge never stuck. However, once I knew the reason why they are needed, everything clicked. So, I decided to write it down.

Understand the difference between routers and switches

Comparing these two devices side by side makes it much easier to understand.

Switch：
　→ Transfer data inside the same network
　→ Use MAC address
　→ Layer 2（Data Link Layer）

Router：
　→ Transfer data in different networks
　→ Use IP Address
　→ Layer 3（Network Layer）

Familiar example

Switch = extension call in the office
　→ Connect with people from the same company

Router = international telephone exchange
　→ Connects people from different countries (networks)
　→ Determines which route to use

Division of roles of routers and switches

　　　　　　        Internet
　　　　　　　　  　    │
　　　　　┌────────────┴──────────┐
　　　　　│      Router           │ ← Layer 3
　　　　　│ Different NW Transfer │ Judge from IP address
　　　　　└────────────┬──────────┘
　　　　　　　　      　│
　　　　　┌────────────┴─────────────┐
　　　　　│            Switch         │ ← Layer 2
　　　　　│ The same NW Transfer      │ Judging from MAC address
　　　　　└───────┬──────────┬────────┘
　　　　　  　  　│　　　     │
　　　　  　   ┌──┴──┐   　┌──┴──┐
　　　　     　│PC-A │　   │PC-B │
　　　　  　   └─────┘　   └─────┘

The main role of routers

1. Routing ( Route Selection )

It decides routes that head to the destination.

Tokyo → Osaka：
  Route A： Tokyo → Nagoya → Osaka
　Route B： Tokyo → Kyoto → Osaka
　　↓
　Choose the best path for data transfer

2. Network segmentation・ border

The router acts as the boundary between the Internet and the internal network.

   　　　　　　Internet
　　　　　　　　　│
　　　　　　　　　│ Global IP
　　　　　　　　　│ 203.0.113.1
　　　　　┌──────┴──────┐
　　　　　│   Router    │
　　　　　└──────┬──────┘
　　　　　　　　　│ Private IP
　　　　　　　　　│ 192.168.1.1
　　　　　┌──────┴──────┐
　　　　　│   Switch    │
　　　　　└──┬───────┬──┘
　　　　　 　│　　  　│
　　　　　┌──┴──┐ ┌──┴──┐
　　　　　│PC-A │ │PC-B │
　　　　　│~.1.2│ │~.1.3│
　　　　　└─────┘ └─────┘

3. NAT ( Network Address Translation)

NAT is a mechanism that allows multiple devices to share a single global IP address.

【NAT（ Sharing one IP address by multiple PCs）】

Internal Network (Office)        Internet
┌───────────────────┐            ┌───────────────┐
│ PC-A 192.168.1.2  │            │               │
│ PC-B 192.168.1.3  ├── Router ──┤  203.0.113.1  │
│ PC-C 192.168.1.4  │            │  （Just one） │
└───────────────────┘            └───────────────┘
Multiplex private IPs           　1 Global IP

4. Data flow

【PC-A → Data flow to the Internet】

PC-A（192.168.1.2）
　↓ " I need to send this to the Internet"
Switch
　↓ head to Router
Router
　↓ exchange to global IP（NAT）
　↓ select the best path
Internet
　↓
Destination server

5. Packet filtering


A basic security feature that blocks unauthorized access 
from outside the network.

Routing Table

Routers have maps called routing table.
They look at it and select the path for destinations.

Destination network  | Next router      | Distance
192.168.1.0/24  | Direct connection        | 0
10.0.0.0/8      | 192.168.2.1     | 1
0.0.0.0/0       | 203.0.113.1     | Default

Static routing and Dynamic routing

Static routing

It is a method to configure the routing tables by hand.

Administrator
　↓ Configure the route by hand
Router A ──→ Router B ──→ Router C
　　　　　　　　　　　　　　　↓
　　　　　　　　　　　　　Destination NW

Static route → If you need change, you have to update it by hand as well

Features：
・For small scale network
・Best suited for environments where routes rarely change 
・Simple settings
・Gives administrators full control over routing decisions

Dynamic routing

It is a method to configure the routing tables automatically.

Router A ←→ Router B ←→ Router C
　↕　　　　　　↕　　　　　　↕
Exchange routing information each other　
Choose the best path automatically 
Handle automatically, if routes change

Features：
・For large scale network
・Best suited for environments where routes change frequently
・Handle automatically, if routes change
・Protocols： RIP・OSPF・BGP etc…

Proper use

Static Routing  → Small scale and the least update environment
Dynamic Routing → Large scale and frequent update environment

Links with OSI reference model

Layer 7  Application  │
Layer 6  Presentation │
Layer 5  Session      │  Not applicable for routers
Layer 4  Transport    │
─────────────────────────────
Layer 3  Network      ← Router function （IP Address）
─────────────────────────────
Layer 2  Data Link    ← Switch function （MAC Address）
Layer 1  Physical     ← Cable・ Physical signal
─────────────────────────────

Familiar example for routers

A home Wi-Fi router is actually an all-in-one device that combines a router, a switch, and a Wi-Fi access point.

Domestic Wi-Fi Router
┌─────────────────────────────────────────────────────────────────┐
│ Router function→ Connect between domestic network and Internet  │
│ Switch function→ Connect PC with wired connection               │
│ Wi-Fi AP function→ Connect devices with wireless connection     │          
└─────────────────────────────────────────────────────────────────┘
　　　　　　　↕
　　     Internet

Summary

Router = Devices that transfer data between different networks

Main role：
　→ Routing（ Selection for the best route）
　→ Border for network
　→ NAT（ Sharing one IP by multiple PCs）
　→ Packet filtering

The difference of Router and Switch：
　Router → Layer 3・IP address・ Between different NWs
　Switch → Layer 2・MAC address・ Between same NWs

Routing kinds：
　Static  → manual・ for small scale
　Dynamic → automatic・ for large scale

OSI Layer： Layer 3（Network Layer）

Conclusion

When I started to study for them, I couldn't remember each functions of these devices.
Once I connected each function to a familiar real-world example, everything fell into place. Understanding the difference between routers and switches makes these types of questions much easier to handle.

Understanding Loopback Adapter Through Its Structure

TAKUMI SUGATA — Mon, 04 May 2026 04:09:48 +0000

Introduction

While studying for CompTIA Network+, I couldn't grasp what a Loopback Plug actually does. I understood that it was used for testing, but had no idea how it worked in practice. As a result, I kept getting questions about it wrong. Once I understood the structure behind it, everything clicked. So I decided to write it down.

What is NIC

NIC stands for Network Interface Card.
A NIC is a component inside a computer that enables it to connect to a network.

Familiar example

Computer = Human beings
NIC      = Ears and mouth

Without ears, we cannot hear voice.
Without mouth, we cannot convey voice.

Similar to these example, computers cannot receive and send signal without NIC.

Physical position

NIC is built-in computers.
The visible part is the port where you plug in a LAN cable - the RJ-45 port.

PC's back or sides
┌───────────────────────────────┐
│  [USB] [USB] [RJ-45] [HDMI]   │
└───────────────────────────────┘
                  ↑
              NIC port
     The hole to insert LAN cable

The role for NIC

1. Electric signal ↔ Digital data
　→ It transfer electric signal in Network to the data that computers can read

2. MAC address management
　→ Each NIC has a unique identifier called a MAC address
　→ The only number in the world

3. Control for sending and receiving
　→ It controls the timing for sending and receiving data

Relationship with OSI reference model

Layer 2  Data Link ← Management for MAC address
Layer 1  Physical  ← Sending and receiving electric signal

NIC involves in both Physical and Data Link layers.

What is Lookback Plug

A loopback Plug is a dedicated testing tool used to verify whether a NIC is functioning correctly.

Normally, other devices is needed to connect to Network, but lookback plug enables us to test for NIC without cables or other devices.

Normal connection：
PC → cable → switch → the other PC

Loopback Plug：
PC → signal loops back to itself → received by itself

Kinds

There are two types in Lookback Plug.

1. RJ-45 Loopback Plug（ for copper wire cables）
2. Fiber Loopback（ for optical fiber）

The structure of RJ-45 Lookback Plug

Inside the RJ-45 connector, the sending pins (TX) and receiving pins (RX) are directly connected.

【RJ-45 Loopback Plug （front）】

┌───────────────────────────────────────────────┐
│  1     2     3     4     5     6     7     8  │
│  TX   TX    RX  　  　         RX      　  　  │
└───────────────────────────────────────────────┘
     │     │ │                   │
     │     └ │ ──────────────────┘    
     └───────┘
    （Connect TX to RX）

The sent signal loops back inside the plug and returns to the receiving pin.

The structure of Fiber Lookback

The Fiber Loopback connects the TX and RX ports using an external cable.

【Fiber Loopback structure】

        ┌────────────────┐
        │                │
   TX ──┤ >>>>>>>>>>>>>> ├── RX
        │ Electric fiber │
        └────────────────┘

   Sent electric signal turn around to the receiving port

【Connector （front）】

  送信側          受信側
┌──────┐   ┌──────┐
│ [TX] │~~~│ [RX] │
│ port │   │ port │
└──────┘   └──────┘
   └──────────┘
Connect with loop cable

What is the test purpose with those adapters

Insert Loopback Plug to NIC port
　↓
Test tools send a signal
　↓
TX （sending pin） sends a signal
　↓
The signal turn around to RX (Receiving pin) within Loopback Plug 
　↓
The same signal comes back
　↓
✓ If it returns → NIC・ port is normal
✗ If it doesn't return → NIC・ port has problems

What specifically can we learn

✓ What we can know：
・Whether NIC can send and receive signal
・Whether physical port breaks
・Whether NIC itself is normal without cables

✗ What we cannot know：
・Cable issues
・Switch issues
・IP address settings issues
・Internet connection issues

How to use in troubleshooting

When PC cannot connect to Network, lookback plug is used to isolate the problems.

Step 1：Test with Loopback Plug
　→ failed → NIC・ port is the cause of problem
　→ success → NIC・ port is not the cause of problem

Step 2：Test with cable
　→ failed → cable is the cause of problem
　→ success → cable is not the cause of problem

Step 3：Test with switch
　→ failed → switch is the cause of problem
　→ success → network settings is the cause of problem

** Isolating the root cause step by step** is the fundamental approach to troubleshooting.

Relationship with OSI reference model

Lookback plug is used for test in *Physical layer (Layer 1) *.

Layer 7  Application    │
Layer 6  Presentation │  Not subjecting to testing
Layer 5  Session           │
Layer 4  Transport       │
Layer 3  Network         │
Layer 2  Data Link        │
─────────────────────────────
Layer 1  Physical     ← Subjecting to testing with Loopback Plug
─────────────────────────────

The standard approach is to confirm Physical Layer first, then work upward through each layer.

Summary

NIC = Computer's components to connect to network
　→ NIC port for LAN cable inserting
　→ Involves in Physical and Data Link layers

Loopback Plug = Plug for testing whether NIC port is normal
　→ Sent signal turn around itself
　→ Kinds：RJ-45・Fiber
　→ What it can test： Physical normalcy in NIC・ port
　→ What it cannot test： cable・ switch・ settings problems
　→ OSI層：Only Physical Layer （Layer 1）

Conclusion

I used to struggle with Loopback Plug because I didn't even know what a NIC was in the first place.
Not knowing what NIC was made it impossible to understand what the plug was actually testing.
Once I understood NIC first and then the structure of the Loopback Plug, everything fell into place.

Understanding Mobile Generation with Netflix

TAKUMI SUGATA — Sat, 02 May 2026 05:10:11 +0000

Introduction

While studying for CompTIA Network+, I couldn't totally remember each speed for mobile generations; 3G・4G・5G.

The numbers alone meant nothing to me - I had no way to visualize what they actually felt like. Once I matched each generation's speed to Netflix streaming quality, everything clicked.

What is Theoretical Max and Typical Speed

There are two kinds of the speed in mobile communication.

Theoretical Max（ Theoretical value）
　→ The max speed under the ideal condition
　→ No obstacles, directly beside the base station, no congestion

Typical Speed（ Typical value）
　→ The average speed in the actual use environment
　→ Inside buildings, far from base station, congestion

The reason why each speed is different

Radio waves interference
　→ Buildings ・ walls ・ obstacles weak signal

Distance from base station
　→ The farther the base station, the weaker the signal

Number of simultaneous connection
　→ Many users sharing the same base station causes speed to be distributed across all connections

Frequency band
　→ 5G, use high frequency (millimeter wave) , is fast, but short range

Understanding familiar example

Theoretical Max = The maximum speed achieved by driving on a highway with no traffic, no traffic lights, and no obstacles

Typical Speed   = The actual average speed in commuting rushing hours

Even the highest-performing standard is affected by factors such as interference, distance, and the number of simultaneous connections.

The speed of each mobile generation

Generation	Theoretical Max	Typical Speed
3G	2Mbps	384Kbsp〜2Mbps
4G（LTE）	1Gbps	30〜150Mbps
5G	20Gbps	100Mbps〜3Gbps

Imagine in Netflix

I know it is too hard to think of the just numbers, so I gave an example about Netflix.

the standard number

Watch Netflix HD videos = Need approximately 5Mbps
Watch Netflix 4K videos = Need approximately 25Mbps

3G (Typical Speed：384Kbps〜2Mbps）

・Netflix HD → Impossible（ Need 5Mbps, but not stable）
・YouTube (low video quality) → Just barely
・Send message → No problem
・Google search → A bit slower

Downloading time for a 1GB file : about 20 minutes ～ 1 hour

This generation spread in 2000s was the first standard that enabled smartphone internet access, but it is no longer in use today.

4G LTE (Typical Speed：30〜150Mbps)

・Netflix HD → Comfortable（ Enough by 5Mbps）
・Netflix 4K → Limited (Need 25Mbps, just barely reached) 
・Video call → Comfortable
・Downloading huge size application → A few minute

Downloading time for a 1GB file : about 3 minutes ～ 7 minutes

LTE stands for Long Term Evolution
This is the most common standard in today's world.

5G（Typical Speed：100Mbps〜3Gbps）

・Netflix 4K → No problem at all - overwhelmingly faster than the required 25Mbps
・8K Video → Possible
・Simultaneously connection with multiple devices → Comfortable
・Downloading 1GB size application → tens of seconds

Downloading time for a 1GB file : about 10 seconds ～ 1 minute

The theoretical maximum of 5G is 20 times faster than of 4G.
But, typical value highly depends on the environments.
This technology is expected to use in the field for IoT, automatic device and smart city.

Array of these three generation

Downloading time of 1GB：
3G  → about 20 minutes 〜 an hour
4G  → about 3 〜 7 minutes
5G  → about 10 seconds 〜1 minute

Features of each generation

・Spread in 2000's 
・This is the generation for the first time in smartphone use with Internet
・No service in today's world

4G LTE

・LTE = Long Term Evolution
・Main communication standard in today's world
・Video streaming is comfortable in this generation
・The most common standard over the world

・Theoretical value is as 20 times as 4G
・Simultaneously connection is as 100 times as 4G
・Latency is approximately 1/30th of 4G (about 1ms)
・Expected to use in the area of IoT ・ automatic drive ・ smart city

Summary

Generation	Theoretical Max	Typical Speed	Netflix HD	Netflix 4K
3G	2Mbps	384Kbps〜2Mbps	✗	✗
4G LTE	1Gbps	30〜150Mbps	✓	△
5G	20Gbps	100Mbps〜3Gbps	✓	✓

How to memorize

It is important to memorize that relative speed in each generation rather than detailed numbers

3G  = Mbps unit（ Cannot totally watch Netflix HD）
4G  = tens Mbps（ Comfortable to watch Netflix HD ・limited to watch 4K video）
5G  = Hundreds of Mbps 〜 Gbps（ Netflix 4K with no issues）

Conclusion

I used to struggle with memorizing the speeds for each generation.
Once I connected them to real-world download times, everything fell into place.
Attaching numbers to real experiences makes them far easier to retain.

Understanding the Evolution of CAT Standards Makes Memorization Unnecessary.

TAKUMI SUGATA — Wed, 29 Apr 2026 04:39:20 +0000

Introduction

While studying for CompTIA Network+, I couldn't remember CAT standards in LAN cable field.
The sheer number of specification made it difficult to answer related questions confidently.
Once I understand how each standard evolved from the previous one, everything clicked.

What is CAT standards

LAN cable has CAT (Category) standards, which shows that higher number perform better.
Understanding the logic behind each standard is more effective than memorizing numbers.

Why higher standard accelerates speed

LAN cables consist of 8 copper wires. Trying to increase signal speed introduced two key challenges.

Problem 1：Crosstalk
Receive electrical interference from the adjacent copper cables
→ Signal degrades

Problem 2：Attenuation 
Longer distance weakens signal
→ Not be able to reach

The higher standards are designed to solve these problems.

What is the solution in each evolution stages

CAT5 → CAT5e

Problem： Frequent Crosstalk
Solution： Twist Copper cables
Result：100Mbps → 1Gbps

Twisting has an advantage to solve signal interference.

Rough twisting → More interference → slower
Fine twisting → Less interference → faster

e shows Enhanced .

CAT5e → CAT6

Problem： Pushing beyond 1Gbps increases susceptibility to Crosstalk
Solution： Introduce spine in a center of the cable and more twist
Result：1Gbps → 10Gbps
※10Gbps reaches under 55 meters

** Why CAT6 cannot reach over 55 meters in using 10Gbps

Faster signal = Increase electrical interference
　↓
Longer distance can degrade the signal due to accumulation of interference
　↓
Over 55m cannot retain the speed of 10Gbps

Without shielding, the cable is susceptible to outside interference.

CAT6 → CAT6a

Problem：CAT6 cannot reach over 55m in 10Gbps
Solution： Wrap the cable by shield (metal shroud), which blocks outer interference
Result：55m (10Gbps) → 100m

Without shield（CAT6）
→Influenced by outer electric devices 
→Longer distance degrades signal

With shield（CAT6a）
→Block outer interference
→Retain longer distance at 10Gbps

a shows Augmented .

CAT6a → CAT7

Problem：CAT6a still has interference at high frequencies between wire pairs 
Solution：Wrap each individual wire pair with its own shield
Result：Crosstalk between pairs reduced to nearly zeeo

The difference between CAT6a and CAT7 lies in the shielding structure.

CAT6a：
┌─────────────────────────────────────┐
│ Whole Shield                        │
│  ○ ○ ○ ○（The pair without shield） │
└─────────────────────────────────────┘

CAT7：
┌───────────────────────────────────┐
│ Whole Shield                      │
│ [○][○][○][○]（Individual shield ）│
└───────────────────────────────────┘

CAT7 → CAT8

Problem： Lacking with 10Gbps for data center
Solution： Enforce the quality of copper cables and shield
Result：10Gbps → 40Gbps, but reach just 30m

The reason why CAT8 can reach just 30m

High speed signal (40Gbps)
　↓
High electrical interference 
　↓
Shield cannot function to retain the signal strength for long distance
　↓
Use under 30m

CAT8 is designed for connecting to server racks inside data center.

Server rack A ──── 30m ────→ Server rack B
　↓
The length is enough for data center 
　↓
It prioritize the speed rather than distance

Why CAT5-CAT6a are the length of 100m

This is the reason for the designed standard.

The typical distance from one end of an office floor to the other is approximately 100m.
Ethernet standard is designed for architectural standard.

The edge of the office ──── 100m ────→ Server room
　↓
Ethernet standard can cover them
　↓
CAT5〜CAT6a = 100m

Understanding whole structure

Need for fast speed
　↓
Problem for interference and attenuation
　↓
Solution①：more twisted（CAT5e・CAT6）
Solution②：cover with whole shield（CAT6a）
Solution③：cover each pair with individual shield（CAT7）
Solution④：enhance the quality of materials and structure（CAT8）
　↓
The faster the speed, the greater the interference challenges become.
　↓
The limited distance for the fastest speed（CAT8の30m）

Summary

Standard	Speed	Distance	Improvement	Use case
CAT5	100Mbps	100m	Normal structure	Traditional environment
CAT5e	1Gbps	100m	Twist Copper cables	Household・Office
CAT6	10Gbps	55m※	Introduce spine in a center of the cable	Enterprise office
CAT6a	10Gbps	100m	Wrap whole cable by shield	Data center
CAT7	10Gbps	100m	Wrap the individual cable by shield	High performance enterprise network
CAT8	40Gbps	30m	Enforce the quality of copper cables and shield	Inside data center

※CAT6: 10Gbps can reach 55m, but 1Gbps can reach 100m.

Key numbers from the exam

CAT6  = 10Gbps → 55m (not 100m)
CAT6a = 10Gbps → 100m
CAT8            → 30m
All others      → 100m

Conclusion

I used to struggle with these questions.
Each standard exists to solve a specific interference problem.
Once you understand that evolution, the numbers take care of themselves