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Aisalkyn Aidarova
Aisalkyn Aidarova

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LAB: Subnetting with Router (2 Networks)

🧠 PART 1 — WHAT YOU BUILT

  • LEFT side → Switch + PCs → Subnet 1
  • RIGHT side → Switch + PCs → Subnet 2
  • Middle → Router → connects them

👉 This is real-world design

🧠 PART 2 — WHY SUBNETTING IS IMPORTANT

❌ Without subnetting (one big network)

  • Too many devices
  • Broadcast storms
  • Slow network
  • No security separation

✅ With subnetting

You create smaller networks (subnets):

Benefit Explanation
Performance Less broadcast traffic
Security Can control access
Organization HR, IT, Finance separated
Scalability Easy to grow

👉 Example (your lab):

  • Subnet 1 = Office A
  • Subnet 2 = Office B
  • Router = gate between them

🧠 PART 3 — SUBNET DESIGN (VERY IMPORTANT)

We define:

Subnet 1 (LEFT) 

Network: 192.168.1.0/24
Gateway: 192.168.1.1
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Subnet 2 (RIGHT) 

Network: 192.168.2.0/24
Gateway: 192.168.2.1
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💡 Explain /24

2^8 = 256

  • 8 bits for hosts
  • 256 IP addresses total
  • 254 usable

🧠 PART 4 — CONFIGURATION (STEP-BY-STEP)

🔹 1. Configure Router

Open Router CLI:

enable
configure terminal

interface g0/0
ip address 192.168.1.1 255.255.255.0
no shutdown

interface g0/1
ip address 192.168.2.1 255.255.255.0
no shutdown
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🔹 2. Configure PCs (VERY IMPORTANT)

LEFT PCs

PC IP Gateway
PC0 192.168.1.10 192.168.1.1
PC1 192.168.1.11 192.168.1.1
PC2 192.168.1.12 192.168.1.1

RIGHT PCs

PC IP Gateway
PC3 192.168.2.10 192.168.2.1
PC4 192.168.2.11 192.168.2.1
PC5 192.168.2.12 192.168.2.1

🧠 PART 5 — HOW IT WORKS (CRITICAL)

🔁 Same Subnet Communication

Example:

PC0 → PC1
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✔ Goes through switch only
❌ Router NOT used

🌍 Different Subnet Communication

Example:

PC0 → PC3
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Steps:

  1. PC0 sees → destination is different subnet
  2. Sends packet to default gateway (router)
  3. Router checks routing table
  4. Router forwards to subnet 2
  5. PC3 receives

👉 THIS is the key concept:

Router = device that connects different networks

🧠 PART 6 — TESTING

🔹 Test same subnet

From PC0:

ping 192.168.1.11
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✔ Should work

🔹 Test different subnet 

ping 192.168.2.10
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✔ Should work only if router configured correctly

🧠 PART 7 — WHAT HAPPENS IF NO ROUTER?

👉 “What if router is removed?”

Answer:

  • PC0 cannot reach PC3
  • Because different networks need routing

🧠 PART 8 — INTERVIEW LEVEL EXPLANATION

You can say:

Subnetting divides a large network into smaller logical networks to improve performance, security, and manageability. Devices within the same subnet communicate directly via Layer 2, while communication between subnets requires a Layer 3 device like a router.

🧪 BONUS TASK

  1. Change subnet mask to /25
  2. Create 4 subnets
  3. Assign new IPs
  4. Test connectivity

🧠 WHAT IS BROADCASTING?

Broadcast = one device sends data to ALL devices in the network

🔁 Example 

PC1 → ALL PCs in same network
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📦 Real case (VERY IMPORTANT)

When PC1 doesn’t know MAC address:

It sends:

“Who has 192.168.1.20?”

👉 This is ARP broadcast

🔹 What happens

  • Switch receives frame
  • Sends it to ALL ports (same VLAN only)

💡 Key rule

Broadcast stays inside the same subnet / VLAN

🔥 In your lab

  • VLAN 10 → broadcast stays inside VLAN 10
  • VLAN 20 → separate broadcast domain

🧠 WHAT IS MULTICASTING?

Multicast = one device sends data to a specific GROUP of devices

🔁 Example 

Server → Only interested clients
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📦 Real examples

  • Video streaming
  • Online classes
  • Stock market feeds

👉 Not everyone receives it — only subscribers

🔹 IP Range for Multicast 

224.0.0.0 – 239.255.255.255
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🧠 WHAT IS UNICAST (IMPORTANT TOO)

Unicast = one-to-one communication

🔁 Example 

PC1 → PC2
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✔ Most common
✔ Used in ping, web traffic, SSH

⚖️ DIFFERENCE (VERY CLEAR)

Type Meaning Example
Unicast 1 → 1 PC → Server
Broadcast 1 → ALL ARP request
Multicast 1 → Group Streaming

🧠 SIMPLE ANALOGY

  • Unicast → Phone call
  • Broadcast → Shouting in a room
  • Multicast → Talking to a group in a meeting

🔥 WHY BROADCAST IS IMPORTANT (AND DANGEROUS)

✅ Needed for:

  • ARP
  • DHCP discovery

❌ Problem:

Too many broadcasts =

👉 Network slowdown
👉 CPU overload
👉 “Broadcast storm”

🧠 HOW VLAN HELPS

👉 VLAN reduces broadcast

Instead of:

1 big network → 1000 devices receive broadcast
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You get:

VLAN 10 → 100 devices
VLAN 20 → 100 devices
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✔ Faster
✔ Cleaner

🧠 HOW ROUTER HANDLES IT

👉 Router does NOT forward broadcast

Example:

  • PC in VLAN10 sends broadcast ❌ Router blocks it ✔ VLAN20 never receives it

🧠 MULTICAST IN REAL NETWORKS

Used when:

  • You don’t want broadcast (too heavy)
  • You don’t want multiple unicast (too expensive)

Example:

Instead of:

Server → 100 users (100 separate streams)
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Multicast:

Server → 1 stream → group receives
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✔ Efficient

🔥 INTERVIEW ANSWER (SHORT)

Broadcast is one-to-all communication within a subnet or VLAN and is used for operations like ARP. Multicast is one-to-many communication where data is sent only to a specific group of devices. Unlike broadcast, multicast is more efficient because it avoids sending data to unnecessary devices.

In your Packet Tracer:

  • ARP request = broadcast
  • Ping = unicast
  • VLAN = controls broadcast domain

🔥 FINAL LINE (MEMORIZE)

Broadcast sends data to all devices in a network, multicast sends data to a selected group, and unicast sends data to a single device.

🧠 PART 1 — HOW COMMUNICATION REALLY WORKS

🔹 1. SAME NETWORK (NO VLAN)

Example:

PC1 (192.168.1.10) → PC2 (192.168.1.20)
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What happens step-by-step:

  1. PC1 checks:
  • “Is 192.168.1.20 in my network?” ✔ YES
  1. PC1 sends ARP:

“Who has 192.168.1.20?”

  1. Switch:
  • Broadcasts to all ports

  1. PC2 replies with MAC

  2. Switch learns:

  • MAC → Port mapping (MAC table)
  1. Data flows directly

👉 Key:

  • Uses MAC address
  • Switch works at Layer 2
  • Router NOT used

🧠 2. DIFFERENT NETWORK (SUBNET)

Example:

PC1 (192.168.1.10) → PC3 (192.168.2.10)
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What happens:

  1. PC1 checks:
    ❌ “Different network”

  2. PC1 sends packet to:

Default Gateway (Router)
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  1. Router:
  • Removes Layer 2 frame
  • Checks IP (Layer 3)
  • Decides where to send
  1. Router forwards to subnet 2

👉 Key:

  • Uses IP address
  • Router is required

🧠 3. VLAN (WHY SWITCH BLOCKS)

Example:

  • PC1 → VLAN 10
  • PC2 → VLAN 20

What happens:

  1. PC1 sends ARP
  2. Switch checks VLAN
  3. ❌ Does NOT forward to VLAN 20

👉 VLAN = separate broadcast domain

🧠 FINAL RULE (MEMORIZE)

👉 Same VLAN → Switch
👉 Different VLAN → Router

🚀 PART 2 — FULL LAB (USING YOUR TOPOLOGY)

🎯 Goal

You will:

  • Create VLANs
  • Assign ports
  • Configure trunk
  • Configure router (inter-VLAN routing)
  • Enable DHCP
  • Add ACL security
  • Test everything

🧪 STEP 1 — VERIFY TOPOLOGY

You already have:

  • PCs ✔
  • Switch ✔
  • Router ✔

Make sure:

  • PCs connected to switch
  • Switch connected to router

🧪 STEP 2 — SWITCH CONFIGURATION

Click Switch → CLI:

🔹 Enter config mode 

enable
configure terminal
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🔹 Create VLANs 

vlan 10
name HR

vlan 20
name IT
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🔹 Assign Ports

Example (adjust to your ports):

interface fa0/1
switchport mode access
switchport access vlan 10

interface fa0/2
switchport mode access
switchport access vlan 10

interface fa0/3
switchport mode access
switchport access vlan 20

interface fa0/4
switchport mode access
switchport access vlan 20
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🔹 Configure Trunk (IMPORTANT)

Port connected to router:

interface fa0/24
switchport mode trunk
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🧪 STEP 3 — ROUTER CONFIGURATION

Click Router → CLI:

🔹 Enter config 

enable
configure terminal
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🔹 Router-on-a-Stick 

interface g0/0.10
encapsulation dot1Q 10
ip address 192.168.10.1 255.255.255.0

interface g0/0.20
encapsulation dot1Q 20
ip address 192.168.20.1 255.255.255.0

interface g0/0
no shutdown
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👉 This allows router to handle VLANs

🧪 STEP 4 — DHCP CONFIG 

ip dhcp pool HR
network 192.168.10.0 255.255.255.0
default-router 192.168.10.1

ip dhcp pool IT
network 192.168.20.0 255.255.255.0
default-router 192.168.20.1
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🔹 On PCs

  • Click PC
  • Desktop → IP Configuration
  • Select DHCP

🧪 STEP 5 — ACL (SECURITY)

🎯 Block HR → IT 

access-list 100 deny ip 192.168.10.0 0.0.0.255 192.168.20.0 0.0.0.255
access-list 100 permit ip any any
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🔹 Apply ACL 

interface g0/0.10
ip access-group 100 in
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🧪 STEP 6 — TESTING

✅ SAME VLAN

From VLAN10 PC:

ping 192.168.10.X
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✔ Should work

❌ BLOCKED

From VLAN10:

ping 192.168.20.X
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❌ Should fail

✅ ALLOWED

From VLAN20:

ping 192.168.10.X
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✔ Should work

🧠 PART 3 — WHAT STUDENTS MUST UNDERSTAND

🔹 Switch Role

  • Forwards based on MAC
  • Works inside VLAN only

🔹 VLAN Role

  • Separates Layer 2 traffic
  • Creates isolated networks

🔹 Subnet Role

  • Defines IP structure
  • Enables routing

🔹 Router Role

  • Connects networks
  • Routes packets

🔹 DHCP Role

  • Assigns IP automatically

🔹 ACL Role

  • Controls traffic (security)

🔥 FINAL INTERVIEW ANSWER

Devices in the same VLAN communicate directly using Layer 2 switching. When devices are in different VLANs or subnets, communication requires a Layer 3 device such as a router. VLANs provide logical segmentation, subnetting provides IP structure, and routing enables communication between networks.

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