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IPv6 DHCP and Router Advertisements - Cisco 010-151 DCTECH Exam Guide

Jimmy Victor - Acedexam — Wed, 11 Sep 2024 08:30:26 +0000

The original design of IPv6 did not include DHCP; all hosts would use SLAAC to calculate their IPv6 address. As noted previously, operators discovered there are still reasons to have a lightweight protocol that supports interface address configuration.

There are some minor differences between DHCP for IPv4 and DHCP for IPv6 (often called DHCPv6):
•Instead of broadcasting responses, the server can send packets to the client’s link local address, thus eliminating all server broadcasts.
•Instead of broadcasting packets intended for the DHCP server, the client sends these packets to a multicast group.
•A server can assign an address to a host based on a DHCP unique identifier (DUID), which is calculated by the client. If the host’s physical address changes, it can keep or recover its previous IPv6 address.
•Many unnecessary and unused options from DHCP for IPv4 have been removed in DHCPv6.
•The discover message in IPv4 DHCP is the solicit message in DHCPv6.
•The offer message in IPv4 DHCP is the advertise message in DHCPv6.
•The acknowledge message in DHCP for IPv4 is the reply message in DHCPv6.

One major difference between DHCP for IPv4 and DHCPv6 is the prefix length and default gateway are not included in the DHCPv6 reply message. Instead, these are carried in a separate IPv6 protocol called Router Advertisements (RAs). Here is the prep course: https://www.acedexam.com/010-151-dctech-supporting-cisco-datacenter-networking-devices/

IPv6-capable routers send RAs to each segment to

•Inform hosts connected to the segment the router can be used as a default gateway.
•Indicate whether hosts connected to this segment should automatically compute their IPv6 addresses via SLAAC or should ask for an address through a DHCPv6 server.
•Inform hosts connected to the segment about the maximum packet size (or maximum transmission unit [MTU]).

Note
The chapters in Part II explore MTU in more detail.

Host-to-Host Communication and Address Resolution on a Single Wire

Once all the hosts connected to a single segment have physical and interface addresses, they can begin to communicate. Figure 3-4 will be used to explain the process.

Figure 3-4 Host-to-Host Communication

In the figure, each host has two addresses: a physical address and an interface address. Host A could send packets destined to host D to the correct interface and broadcast physical addresses. Host D would certainly receive and process the packet in this case, but host B would need to receive the packet, examine it, determine it does not need to accept or process it, and then discard it. If every host on the segment must receive and process every packet—even if the processing is just to discard the packet—this would be a huge waste of resources.

It is much more efficient if host A can send packets to host D’s correct interface and physical addresses. To do this, however, A must know the relationship between these two addresses; it must resolve host D’s interface address to a physical address reachable on this segment.

IPv4 and IPv6 use different address resolution techniques.

Assign an Address Through a Protocol - Cisco 010-151 DCTECH Exam Guide

Jimmy Victor - Acedexam — Wed, 11 Sep 2024 08:30:24 +0000

Many operators deploy the Dynamic Host Configuration Protocol (DHCP) to
•Support IPv4 hosts, which do not have any way to automatically calculate interface addresses
•Support naming services (DNS)
•Log and control the mapping of interface IP addresses, including controlling who can be assigned an IP address on the network
•Control the assignment of the default gateway
•Ensure the same IP address is not used in multiple places

IPv4 DHCP

Figure 3-3 illustrates the IPv4 DHCP process.

Figure 3-3 IPv4 DHCP Operation

In this figure, host A is connected to the network and needs to obtain an IPv4 address. A, the DHCP client, sends a discover message to determine whether a DHCP server is connected to the network. If no DHCP server answers, A will not be able to obtain an IPv4 address. Get the new study guide at https://www.acedexam.com/010-151-dctech-supporting-cisco-datacenter-networking-devices/

A sends this discovery message to a broadcast address, so router C and host D also receive the message. Since neither is a DHCP server, they will not respond to the message.

Most routers can be configured as DHCP servers; the two functions have been separated in this figure for clarity.

The DHCP server B will examine its local DHCP table for A’s physical address. If the server has assigned A an address in the recent past, it will have a record associating A’s physical address with an already assigned IPv4 address. If so, B will use the existing assignment. Otherwise, B will look for an unassigned address in its address pool and create an entry for A.

Server B offers this IPv4 address to A; the offer message is sent to the broadcast address. Both D and C receive this message, but the physical address of the requesting host (A in this case) is carried in the offer packet, so C and D ignore this message.

When it receives the offer message, A will probe the network using the Address Resolution Protocol (ARP), discussed in more detail in the next section, to verify the address is not in use by another host. After verifying the proposed address is unused, A will send a request message to the DHCP server, B.

The request message is also a broadcast, so C and D receive the packet. Since neither one of these devices is a DHCP server, however, they will ignore the request message.

The DHCP server will then respond with an acknowledgment sent directly (not as a broadcast) to the client, A.

This final acknowledgment message contains other information, such as the default gateway the host should use, the prefix length for this segment, a list of name (DNS) servers available on the network, and a lease time.

The lease time allows IP addresses to be reused if a host has not been active for some time—usually measured in hours or days. Just before a lease expires, the client can request an extension.

Timing out IP address assignments allows addresses to be reused once a host moves permanently to another location in the network.

Computing a Global Address - Cisco 010-151 DCTECH Exam Guide

Jimmy Victor - Acedexam — Wed, 11 Sep 2024 08:30:23 +0000

Once the link local address is calculated, a host running IPv6 can use Stateless Address Autoconfiguration (SLAAC) to calculate a globally routable address. To calculate a global address using SLAAC,The IPv6 software sends a Router Solicitation (RS) message to a special router-only multicast address.

A router on the link will respond with a Router Advertisement (RA). The RA contains the segment prefix.

The IPv6 software combines the EUI-64 address, calculated above, with the segment prefix.

It performs duplicate address detection with this new address to make certain no other host on the segment has the same address.

It sets the default gateway address on the local host to the RA message source.

Note

If the host does not receive an RA in response to its RS message, there are no routers on the segment; hence, there is no way to communicate with any hosts on some larger network or the global Internet. If there are no routers, the host can assume every other host it can reach will be reachable using the link local address.

SLAAC is widely but not always used for configuring IPv6 interface addresses. Operators often note three problems with SLAAC:
•Naming services cannot be configured through SLAAC.
•It is difficult to relate an IPv6 address to an individual user or host in network management systems.
•SLAAC reveals potentially private information in some situations.

It is worth looking at the last item in this list in a little more detail. You can find more details at: https://www.acedexam.com/010-151-dctech-supporting-cisco-datacenter-networking-devices/

Physical Addresses and Privacy

Suppose you have a host—a laptop, mobile phone, or some other device—you use in several locations; Figure 3-2 illustrates.

Figure 3-2 Privacy and Host Movement

In Figure 3-2, a host moves from a home network at 1 to a coffee shop at 2. If the host uses SLAAC to calculate an IP address at both locations, server A can tell this is the same host in two locations because the lower 64 bits of the address will be the same.

In fact, no matter where you take this host, if you attach it to the global Internet and access this same server, server A will be able to know this is the same host. It is possible to track an individual user by noting their IPv6 address everywhere they go. This is a violation of the user’s privacy.

One solution to this problem is configuring the host to use a random physical address. Chapter 11, “Local Area Networks,” discusses this solution in more detail.

A second solution is to assign hosts addresses rather than calculating the interface address from the physical address.

IPv4 Address Resolution - Cisco 010-151 DCTECH Exam Guide

Jimmy Victor - Acedexam — Wed, 11 Sep 2024 08:29:33 +0000

If A, B, and D are running IPv4, the Address Resolution Protocol (ARP) maps physical to interface addresses. When host A wants to send a packet to D:
Host A examines its local ARP cache to see whether it already knows the physical and interface addresses for D.

Given A does not have an existing mapping, it will send an ARP request to the physical broadcast address, so both B and D receive this packet.

The ARP request will have host D’s interface address in the Target Protocol Address field of the ARP packet and host A’s physical address in the Sender Hardware Address field.

When B receives this packet, it will determine its local interface address does not match the target protocol address, so it will discard the ARP packet.

When D receives this packet, it will determine its local interface address matches the target protocol address, so it will build a response.

In its response, host D will include its interface address in the Sender Protocol Address field, and the physical address for the correct interface in the sender hardware address field.

When A receives this response, it can add the mapping between D’s interface and physical addresses, allowing it to send unicast packets to D. More details is at https://www.acedexam.com/010-151-dctech-supporting-cisco-datacenter-networking-devices/

ARP can also be used to notify all the hosts on a segment about an address change or to announce the connection of a new host to the segment. This is called a gratuitous ARP because it is an ARP response that does not correspond to any ARP request. Most hosts will send a gratuitous ARP when they connect to a segment so that all the other hosts will have their interface to physical address mapping in their local cache, saving time and effort in transmitting packets.

Duplicate Address Detection is another function of ARP in an IPv4 network. A host can send an ARP probe to determine whether any host on the segment is already using an IP address. If the host does not receive an answer, it can assume the IP address is not in use.

IPv6 Address Resolution

If A, B, D, and E are running IPv6, the Neighbor Discovery (ND) protocol maps physical interface addresses. One major difference between IPv4 and IPv6 is that IPv6 hosts do not assume every other host on the segment uses the same prefix. Figure 3-5 illustrates.

Figure 3-5 IPv6 Neighbor Discovery
Because of the role the router plays in IPv6 address resolution, we have to start a few steps back from where we started with IPv4:
Router C sends an RA with a list of prefixes.

Each prefix in use on this segment is marked with the L bit.
Each host on the segment—A, B, D, and E—keeps a list of the prefixes in use on this segment based on all the RAs they have received.

When host A wants to send a packet to B:
Host A examines its local list of prefixes in use on this segment.

If host B’s IPv6 address is contained within one of the prefixes on this segment, A sends a Neighbor Solicitation packet to a multicast address.

Host B responds with a Neighbor Advertisement packet linking its IPv6 and physical addresses.

Host A receives the neighbor advertisement and uses it to build a local cache of IPv6 to physical address mappings.

Switching Packets - Cisco 010-151 DCTECH Exam Guide

Jimmy Victor - Acedexam — Wed, 11 Sep 2024 08:29:16 +0000

Up to this point, we have considered hosts connected to a single segment. What if you want to connect multiple segments (or broadcast domains or wires)? There are three ways to connect segments in a computer network:
•Switches
•Routers
•Gateways

Switches act on the physical (Layer 2) interface. Routers act on the interface (Layer 3) address. Gateways or proxies act on some higher-level address, including the protocol identifier and port number. Gateways are outside the scope of this book.

Switches are the simpler of the two kinds of devices we want to look at, so we’ll start there. Figure 3-6 and the list that follows help to illustrate how a switch works.

Figure 3-6 Switching Packets

In Figure 3-6, if host A does not have any information on E, but A wants to send a packet to E:
Host A sends an address discovery packet for E. For IPv6, this will be a neighbor solicitation, and the packet’s destination address will be a multicast. For IPv4, this will be an ARP, and the packet’s destination will be a broadcast.

Switch C receives this packet. Because C receives this packet on port 1, C will learn A is connected to (or reachable by) port 1. A switch learns about which hosts are connected where by examining packets it receives in the normal course of the network’s operation. This is called bridge learning.

Switch C examines the destination address and discovers it is either a multicast (IPv6) or broadcast (IPv4). Broadcast and multicast packets should be forwarded out all unblocked ports, so C forwards this packet through port 2.

Host E receives the address resolution packet and responds.
When C receives E’s response on port 2, it learns E is reachable through port 2.

Switch C forwards E’s responses back through port 1, where A receives the response and builds a local table mapping E’s interface address to E’s physical address.

Note

Switches can decide not to forward a multicast packet out through a port if the switch knows there are no hosts listening to the multicast group. How the switch knows this is outside the scope of this book, but it involves the switch snooping on Internet Group Message Protocol (IGMP) packets.

When A sends a packet toward E, it will place E’s interface and physical addresses into the packet and transmit it onto the segment. When C receives this packet, it will examine its local table, called a bridge or forwarding table, and find E is reachable through port 2. Because the destination is on a different port than where C received the packet, C will forward the packet out the correct port (port 2 in this case). The full guide is at: https://www.acedexam.com/010-151-dctech-supporting-cisco-datacenter-networking-devices/

Note

Hosts will receive and process all packets with a broadcast physical address, some packets with a physical multicast address, and unicast packets only if the destination address matches the physical address of the interface. Switches, on the other hand, receive packets promiscuously, which means they receive and process every packet transmitted on the physical wire or segment.

The process of sending a packet from A to E seems to be just the same as it was without switch C in the network, so what purpose does the switch serve? Let’s walk through the process of A sending packets to B to see the difference:
Host A sends an address discovery packet for B. For IPv6, this will be a neighbor solicitation, and the packet’s destination address will be a multicast. For IPv4, this will be an ARP, and the packet’s destination will be a broadcast.

Host E receives the address resolution packet and does not respond because the packet request does not contain E’s interface address.

Host B receives the address resolution packet and responds with a unicast packet directly to host A.

When C receives B’s response on port 1, it learns B is reachable through port 1.

Host A receives B’s response and creates a local cache entry mapping B’s physical and interface address.

When A sends packets to B, switch C will receive these packets. When C looks up the packet’s destination in its local forwarding table, it will find B is reachable through port 1, which is the same port B itself is reachable through. Because the packet is received on the same port through which the destination is reachable, C does nothing with the packet.

Because C ignores the packet, E never receives it. For a single packet, the reduction in processing load might be small. Breaking up the network into parts greatly impacts the size of buildable networks—or the possible scale.

Internet Protocol Version 6 - Cisco 300-430 ENWLSI Exam Guide

Jimmy Victor - Acedexam — Mon, 09 Sep 2024 12:03:02 +0000

By the 1980s, the global Internet was growing quickly enough that it became obvious more IP address space would be needed. While several schemes to resolve this problem were proposed, only two are widely deployed today: IPv6 and Network Address Translation (NAT).

Note
IPv6 is completely different than IPv4, but we are only concerned with the changes in addressing here. Other changes between IPv4 and IPv6 will be considered in Chapter 14.

The term IP is used when both IPv4 and IPv6 are intended throughout this book.

IPv6 was initially accepted as a draft standard by the Internet Engineering Task Force (IETF) in December 1998, and the first IPv6 addresses were allocated in July of 1999. IPv4 and IPv6 will likely co-exist in most networks for a long time.

In designing IPv6, the IETF quadrupled the address space. Rather than 32 bits divided into four one octet sections, the IPv6 address is 128 bits divided into 16 sections. Each section, sometimes called a quartet, represents two octets of the address using four hexadecimal digits. Figure 2-9 illustrates an IPv6 address.

Figure 2-9 An IPv6 Address

IPv6 addresses include a prefix length to differentiate between the prefix and subnet addresses—just like IPv4—but the maximum prefix length is now /128 rather than /32. Longer addresses are more difficult to work with, but IPv6 addressing is also simplified in some ways:

•Individual hosts always receive a /64 address, and links between network devices normally receive a /128 address. Prefix lengths between /64 and /128 are extremely uncommon.

•The shortest prefix most networks will be allocated will be a /48. Larger companies and service providers may have access to address space with a prefix length as short as a /29, but most of the addresses you will be working with daily will have prefix lengths longer than /48.

•Any single long string of 0s can be replaced with a double colon or :: (you can use the :: only once in an address).

•All leading 0s are omitted.

These simplifications mean you will mostly work with addresses with prefix lengths between a /48 and a /64, or about 16 possible lengths. Much like IPv4 addresses, the simplest way to work with IPv6 prefix lengths—if you insist on working with IPv6 addresses by hand—is by using skips, as shown in Table 2-3. You can find the study guide at https://www.acedexam.com/300-430-enwlsi-implementing-cisco-enterprise-wireless-networks/

Table 2-3 IPv6 Address Skips

For instance, for 2001:db8:3e8::/48 prefix:
•You can create two /49 subnets, 2001:db8:3e8::/49 and 2001:db8:3e8:8000::/49.
•You can create four /50 subnets, 2001:db8:3e8::/50, 2001:db8:3e8:4000::/50, 2001:db8:3e8:8000::/50, and 2001:db8:3e8:c000::/50.
•2001:db8:3e8:500::/54 is not a valid prefix; you count by fours in the second digit for /54s, and 5 is not a multiple of 4.
Just like in IPv4, the first and last address of the subnet are broadcast addresses.

Three further points:
•After working with IPv6 addresses for a while, you will probably recognize common prefix lengths and where their prefixes begin and end.
•Most network operators carefully plan their addressing so only a few prefix lengths are used; this simplifies becoming familiar with them and makes spotting mistakes easy.
•While working with IPv6 addresses, you should use a subnet calculator and/or cheat sheet to prevent mistakes.

Calculating Prefixes Using Skips - Cisco 300-425 ENWLSD Exam

Jimmy Victor - Acedexam — Mon, 09 Sep 2024 12:00:48 +0000

You do not need to memorize the chart, however, if you add one more bit of math to the process we used to calculate the prefix and broadcast address in the preceding section. To understand this method, you need to understand why the skip chart works. Figure 2-8 illustrates.

Figure 2-8 Binary Places in the IPv4 Address

Notice the numbers below each bit; these are the binary places, which are just like the 1s, 10s, 100s, etc., in the decimal number system everyone learns in school. If any of these change to either a 0 or 1, the entire number changes value by the amount shown below:

These numbers are the powers of two from 20 to 27.

Counting over the number of bits in the prefix length—26—we come to the second bit in the fourth octet, which is a 1. If this bit changes to a 0, the value of the number changes by 64, so 64 is the skip value. Networks with a 26-bit prefix length can exist only on boundaries of 64—0, 64, 128, and 192—with a 26-bit prefix length. Because the 26th bit is in the fourth octet, the networks will count by 64s in the fourth octet. The study guide is at https://www.acedexam.com/300-425-enwlsd-designing-cisco-enterprise-wireless-networks/

If you can find the correct octet from the prefix length and then figure out what the skip is, you can calculate the prefix and broadcast address without the chart. Using 198.51.100.70/26 as an example again:

Divide 8 into the prefix length; ignore the remainder and add 1. In this case, 26/8 is 3; we add 1 and find we are working in the fourth octet of the IPv4 address.

Multiply the working octet by 8; subtract the prefix length. In this case, 8*4 is 32, and subtracting 26 from 32 gives us 6.

Find the power of 2 of this number; in this case, 2^6 is 64. Find the prefix. In this case, 64 will go into 70 once, and we’re working in the fourth octet, so the prefix is 198.51.100.64.
Subtract 1 from the skip and add it to the prefix to find the broadcast address. In this case, the skip is 64. Subtracting 1, we get 63, and adding to 64, we get 127, so the broadcast address is 198.51.100.127.

Again, this method takes some practice to remember all the steps, but it reduces the entire problem to some simple division (without remainders), multiplication, addition, and subtraction. With some practice, you can use this technique to quickly find prefixes and broadcast addresses.

What an IP Address Represents - Cisco 300-420 ENSLD Study Guide

Jimmy Victor - Acedexam — Mon, 09 Sep 2024 11:57:15 +0000

Throughout most of the computer networking world, the host and interface addresses are used interchangeably, but they are not really the same thing. In fact, host addresses do not exist in IP networks:
• Each host on an interface has an independent IP address.
• Each interface is (generally) on a separate segment or in a different broadcast domain.

Many protocols and applications will use one of the available IP addresses as a unique identifier. Hosts with only one interface will have only one interface address, and that interface address may be used to identify the host.

When you read or hear the term host address in an IP networking context, it is probably describing an interface address.

Calculating Prefixes and Subnets Using Subnet Masks

The earliest use of IPv4 addresses relied on the subnet mask rather than the prefix length to differentiate the prefix from the subnet address. Figure 2-7 illustrates.

Figure 2-7 The Subnet Mask
An IPv4 address and prefix length are shown on A in Figure 2-7. B is this same IPv4 address translated to four binary octets. C is 32 binary digits laid out as four octets, just like A. In C, the number of 1s, starting at the left, is given by the prefix length, so there are twenty-six 1s, leaving six 0s. The 1s are the network part of the address or prefix; the 0s are the subnet part of the address.The prep course can be found at https://www.acedexam.com/300-420-ensld-designing-cisco-enterprise-networks/

To find the prefix, use a Boolean logical AND, setting the digit in the result, D, to 1 when the digits in both B and C are 1, and setting the digit in D to 0 if the two digits do not match. The resulting four octets in D are converted to a standard decimal IPv4 address.

The prefix—and the network address—in this example is 198.51.100.64.
Seeing the address laid out in binary helps make more sense of the meaning of all the 1s and all the 0s broadcast addresses. If we set the entire subnet portion of the address to 0s, the resulting IPv4 address is 198.51.100.64. This is not only the prefix but also the first of the two broadcast addresses. Setting the subnet portion to 1s results in the last octet translating to 127, so the second broadcast address is 198.51.100.127.

Calculating Prefixes and Subnets Using a Skip Chart
Converting numbers to binary, running Boolean operations, and then converting them back to decimal is time-consuming; using a skip chart to calculate the prefix and broadcast addresses is much faster. Table 2-2 will be used to illustrate the process.

Table 2-2 IPv4 Networks by Prefix Length
Prefix Length Skip Working Octet
8 1 First
9 128 Second
10 64 Second
11 32 Second
12 16 Second
13 8 Second
14 4 Second
15 2 Second
16 1 Second
17 128 Third
18 64 Third
19 32 Third
20 16 Third
21 8 Third
22 4 Third
23 2 Third
24 1 Third
25 128 Fourth
26 64 Fourth
27 32 Fourth
28 16 Fourth
29 8 Fourth
30 4 Fourth
31 2 Fourth
Let’s use the same address—198.51.100.70/26—to calculate the prefix and broadcast address:

Find the prefix length by going down the left column.
Divide the number in the skip column next to the prefix length into the number in the working octet indicated in the third column. In this case, the skip is 64, and we are working in the fourth octet, so we divide 70 by 64.

Ignoring any remainder, multiply the result by the number in the skip column. In this case, 64 goes into 70 once, so we multiply 64 by 1, with a result of 64.

Make the working octet the result; this is the network address. In this case, the network address is 198.51.100.64.

Add the skip to the resulting number and subtract 1; this is the broadcast address. In this case, the skip minus 1 is 63, so we add 63 to 64. The result is 127, so the broadcast address is 198.51.100.127.

Using a skip chart requires a little practice, but it is much faster. If you memorize the chart, you can probably calculate IPv4 prefixes and broadcast addresses without any paper, pen, or computer.

Why Two Addresses? - Cisco 300-425 ENWLSD Study Guide

Jimmy Victor - Acedexam — Mon, 09 Sep 2024 11:55:30 +0000

If every host, camera, television, and toaster already has unique physical addresses, why should we assign interface addresses as well?

The physical address identifies the host, while the interface address describes the topological location of the host. The physical address is a permanent, fixed address every other host attached to the same physical network can use to communicate with it. The interface address, on the other hand, tells other devices where the host is connected to the network or where to send packets if they are not attached to the same physical link.

Another way this might be expressed is the physical address is the address on this wire, while the interface address is the host’s location on this network. The meanings of on this wire and on this network have, as with most terms in the computer network, broadened over time. The prep course of this exam is at https://www.acedexam.com/300-425-enwlsd-designing-cisco-enterprise-wireless-networks/

Yet another way to express the difference between the physical and interface addresses is using the idea of network stack layers, a topic that will be considered in more detail in Chapter 6, “Network Models.” The physical address is commonly called a Layer 2 address, and the interface address is often called a Layer 3 address.

Why not make the physical interface match the interface address? There are network systems where both the physical and interface address are the same. The Open Systems Interconnection network protocol suite, which includes Connectionless Network Protocol (CLNP) and the Intermediate System to Intermediate System (IS-IS) protocols, is designed so a single manually assigned address is used for all the interface and physical addresses.

On the other hand, most network protocols, such as IP, assume a host will need to discover interface addresses once it is attached to the network. If the interface address must be configured to create the physical address, the interface address must be configured before the host can communicate at all—not even with an automatic configuration system. Figure 2-6 illustrates the problem.

Figure 2-6 Address Assignment Bootstrap Problem

There are many ways to solve this bootstrap problem, but the simplest is to make certain the physical address of each device attached to a network is globally unique.

Why not make the interface address match the physical address? Because the interface address is topological, it must also be hierarchical. Rather than being a single flat address space, there must be something like the equivalent of a street number, street name, city, etc., so the address can be aggregated or summarized. Without some form of aggregation, the address of every host in the world would need to be known to every other host in the world—a completely unworkable situation.

Instead, just like in physical shipping, a packet is carried toward its destination in stages, with different parts of the interface address used at different places.

Internet Protocol Version 4 - Cisco 300-420 ENSLD Study Guide

Jimmy Victor - Acedexam — Mon, 09 Sep 2024 11:53:56 +0000

Internet Protocol Version 4
The physical address is just the first of (at least) three layers of addresses used in networking. The next layer up is the interface address, which describes the topological location of the host on the network. There are many kinds of interface addresses, but the two most common are Internet Protocol version 4 (IPv4) and Internet Protocol version 6 (IPv6). This section considers IPv4; the following section will consider IPv6.

Back in 1966, when computer networks were just being developed, Vinton Cerf and Robert E. Kahn started working on the Transmission Control Program to transfer data. They soon realized having a single protocol to control errors, control data flow, provide the information needed to carry data through the network, and insulate host-to-host data transmission from the physical medium would be too large and inflexible. To resolve this problem, they divided the protocol into two protocols called the Transmission Control Protocol (TCP) and the Internet Protocol (IP). The prep course is at https://www.acedexam.com/300-420-ensld-designing-cisco-enterprise-networks/

Note
Chapter 14, “Network Transport,” considers IP and TCP in more detail; this section just considers IP addresses.

An IPv4 address is 32 bits and is split into four decimal sections for ease of writing and reading, as shown in Figure 2-5.

Figure 2-5 IPv4 Address
The IPv4 address is divided into two parts: the prefix and the subnet. The division between these two parts was originally set by the first octet of the address itself:
 If the first octet was between 0 and 127, the address was in the class A range. For class A addresses, the prefix is one octet (the first section of the address), and the subnet part is the remaining three octets of the address.
•If the first octet was between 128 and 191, the address was in the class B range. For class B addresses, the prefix is two octets, and the subnet part is the two remaining octets.
•If the first octet was between 192 and 223, the address was in the class C range. For class C addresses, the prefix is three octets, and the subnet part is the remaining octet.
In 1993 these address classes were replaced with Classless Interdomain Routing (CIDR). Individual IPv4 addresses are always given with a prefix length indicating the dividing point between the prefix and the subnet.
Note
You will hear the parts of the IPv4 address called many different things. The prefix is often called the network or reachable destination, and the subnet is often called the subnetwork, network, or host. The host address can mean the subnet, or the individual address assigned to an interface. Some of these terms have meaning within specific historical contexts that generally do not apply to classless IPv4 addresses. Others have overlapping—and hence confusing—meanings.
To avoid confusion, the two parts of both IPv4 and IPv6 addresses will be called the prefix and subnet throughout this book.

To understand the difference between the prefix and subnet, let’s go back to the four groups of addresses based on their topological reach:
•An interface (or host) with the same IPv4 prefix and prefix length is within the same segment or broadcast domain.
•An interface (host) with a different IPv4 prefix or prefix length is not in the same segment. These hosts are someplace else on this network or they are in a group of networks outside this network.
From the perspective of the host, there is no way to tell the difference between addresses someplace else on this network and addresses outside this network because of aggregation, discussed in a later section of this chapter.
We can define the prefix and subnet as
•The prefix indicates which subnet.
•The subnet is a group of interfaces, hosts, or subnets.

The prefix length is just what it sounds like—the number of bits in the prefix. For IPv4 addresses, the prefix length can only be between 1 and 32 because there are only 32 bits in an IPv4 address. For example:
•10.0.0.0/8: The first 8 bits, or the first octet, are the prefix; the remaining three octets are addresses within the subnet. The first address in the subnet is 10.0.0.0; the last address in the subnet is 10.255.255.255.
•10.1.0.0/16: The first 16 bits, or the first two octets, are the prefix; the remaining two octets are the subnet. The first address in the subnet is 10.1.0.0; the last address in the subnet is 10.1.255.255.
•10.1.1.0/24: The first 24 bits, or the first three octets, are the prefix; the remaining octet is an address within the subnet. The first address in the network is 10.1.1.0; the last address in the network is 10.1.1.255.
The prefix and subnet parts of the address are not always conveniently divided at a dot like the ones in these examples. For example:
•192.0.2.64/27: The first 27 bits, or the first three octets and 3 of the bits in the fourth octet, are the prefix; the remaining 6 bits are addresses in the subnet. The first address in the subnet is 192.0.2.64; the last address in the subnet is 192.0.2.91.
•10.128.192.0/18: The first 18 bits, or the first octet and 2 bits of the third octet, are the prefix; the remaining 14 bits are addresses in the subnet. The first address in the subnet is 10.128.192.0; the last address in the subnet is 10.128.192.255.
As shown in the example of 192.0.2.64/27, an IPv4 prefix can contain 0s. In the subnet portion of the address, however, all 0s and all 1s addresses are considered broadcast addresses or subnet broadcast addresses. Sending a packet to either of these broadcast addresses means every host within the segment or broadcast domain should receive and process the packet. The broadcast addresses for these examples are
•10.0.0.0/8: 10.0.0.0 and 10.255.255.255
•10.1.0.0/16: 10.1.0.0 and 10.1.255.255
•10.1.1.0/24: 10.1.1.0 and 10.1.1.255
•192.0.2.64/27: 192.0.2.64 and 192.0.2.91
•10.128.192.0/18: 10.128.192.0 and 10.128.255.255

The broadcast addresses are the first and last addresses in the prefix.
Note
The all 0s address, or the subnet address itself, is almost never used as a broadcast address. While you should be aware this broadcast address exists, and how to calculate it, when you see “broadcast address,” you should almost always interpret this to mean the all 1s address, or the last address in the prefix.
The all 0s and all 1s addresses, 0.0.0.0 and 255.255.255.255, are also broadcast addresses.
There are at least three ways to find the prefix and subnet addresses. Each section explains one of these three methods, starting from the most difficult to calculate and easiest to understand.

Physical Addresses - Cisco 300-415 ENSDWI Study Guide

Jimmy Victor - Acedexam — Mon, 09 Sep 2024 11:49:01 +0000

As the first chapter noted, physical addresses originally represented a single physical interface on a host or other network device. As computing power increased, developers built several virtual computers, or virtual machines (VMs), on top of a single physical computer.

These VMs needed their own physical addresses so they could send and receive network frames, so virtual interfaces were created. The idea of a virtual interface, once invented, was applied to many other problems; virtual interfaces are now ubiquitous in computer networks.

Note
VMs were originally developed to allow many different users to time-share on a single large-scale computer, such as a mainframe or minicomputer. Developers transferred the idea of VMs from these larger computers to smaller computers (microcomputers, which we call desktop computers today) to build sandboxes and emulators. To play an arcade game on a computer, you need an emulator, which is essentially a VM. If you want to test code to make certain it does not contain a virus, running it in a sandbox, another kind of VM, is a good idea.

The term mainframe originated in the telephone industry. Engineers constructed large frames to hold the massive wiring, crossbar switches, and Strowger switches, required to build a telephone exchange. The frame at the center of a region was called the main frame and housed in the central office. Smaller frames called building distribution frames (BDFs) might be placed in larger buildings as well. The first large-scale computers relied on massive wiring and hence were built using frames like those used in building telephone networks; hence, the term mainframe bled over from the telephone to the computing world.

There are many kinds of physical hardware addresses, but the most common is the Institute of Electrical and Electronics Engineers (IEEE) EUI-48 format, illustrated in Figure 2-4.

Figure 2-4 The EUI-48 Address Format

Note
You might see the term MAC-48 address from time to time. MAC-48 is an older name for EUI-48; the IEEE has declared MAC-48 obsolete.

The EUI-48 address is 48 bits or 6 octets. Each octet is encoded as a pair of hexadecimal digits and often (though not always) displayed in sections divided by dashes. The guide is at https://www.acedexam.com/300-415-ensdwi-implementing-cisco-sd-wan-solutions/

Note
Octet and byte are often used interchangeably in information technology, but they are not always the same thing. A byte is the number of bits a given processor can hold in internal registers or can process at one time. In an 8-bit processor, a byte is 8 bits; in a 32-bit processor, a byte is 32 bits. An octet, on the other hand, is always exactly 8 bits. Byte, however, is often used to mean exactly 8 bits, regardless of the processor. Because these terms have overlapping meaning, you might need to verify which meaning is intended. Byte almost always means a set of 8 bits in networking documentation and standards.

A physical shipping address has multiple parts: recipient, house number, street name, city, region, and state. As noted in the first chapter, part describes a different geographic region.
The EUI-48 address format is broken up in the same way, but rather than describing different geographic regions, each part describes something about the address.
The eighth bit of the first octet is called the I/G bit. The I/G bit tells you what the scope of this address is. If the I/G bit is set to 0, this is a unicast address—an address of a single physical interface. If the I/G bit is set to 1, this is the address of a group of physical interfaces, or a multicast group.
Interfaces are never assigned an EUI-48 multicast address.

Interfaces are programmed to listen to these addresses by software; any individual host might or might not be listening to a particular multicast address.

The seventh bit of the first octet is called the U/L bit. The U/L bit tells you if the address is globally or locally unique. Globally unique means just what it sounds like: no other device in existence, even in space, should have this same address. Locally unique addresses were often assigned by network administrators way back in the mists of time.
The first half, or three octets, of the address, is the organizationally unique identifier (OUI). While the OUI is divided into a few different registries, the main thing you need to know is the OUI tells you who—the organization—assigned the address. If the U/L bit is set to 0, this address was assigned by the device’s manufacturer.
Globally unique numbers are globally unique because each manufacturer is given a block of addresses. Manufacturers assign a number from their pool of addresses to each device they build. So long as these manufacturers assign each number in their pool to precisely one device, every device made will have a unique address.
Note
Could we run out of EUI-48 addresses? In theory, yes, but it does not seem likely any time soon. Even with the two reserved—U/L and I/G—bits removed from the calculation, the EUI-48 address space has some 70 trillion possible addresses. If we do reach the end of the EUI-48 address space, it is possible to recycle older addresses, because devices generally have some expected lifetime. Most devices will be thrown away within 10 or 15 years of being manufactured.
Because the I/G and U/L bits are placed at the end of the first octet, you can always tell what kind of EUI-48 address you are working with by looking at the last digit of the first octet:
•If the first octet ends in a 0, 4, 8, or C, this is a globally unique unicast address.
•If the first octet ends in 1, 5, 9, or D, this is a globally unique multicast address.

There is a longer version of the EUI-48 address called, naturally enough, EUI-64. The EUI-64 address has the same format as an EUI-48 address, only two octets longer—or 64 bits.

The Java Ecosystem - 1Z0-829 Guide

Jimmy Victor - Acedexam — Sat, 07 Sep 2024 02:29:00 +0000

Since its initial release as Java Development Kit 1.0 (JDK 1.0) in 1996, the name Java has become synonymous with a thriving ecosystem that provides the components and the tools necessary for developing systems for today’s multicore world. Its diverse community, comprising a multitude of volunteers, organizations, and corporations, continues to fuel its evolution and grow with its success. Many free and open source technologies now exist that are well proven, mature, and supported, making their adoption less daunting. These tools and frameworks provide support for all phases of the software development lifecycle and beyond.

There are different Java platforms, each targeting different application domains:
•Java SE (Standard Edition): designed for developing desktop and server environments
•Java EE, also known as Jakarta EE (Enterprise Edition): designed for developing enterprise applications
•Java ME (Micro Edition): designed for embedded systems, such as mobile devices and set-top boxes
•Java Card: designed for tiny memory footprint devices, such as smart cards
• More information is at https://www.acedexam.com/1z0-829-java-se-17-developer/

Each platform provides a hardware/operating system–specific JVM and an API (application programming interface) to develop applications for that platform. The Java SE platform provides the core functionality of the language. The Java EE platform is a superset of the Java SE platform and, as the most extensive of the three platforms, targets enterprise application development. The Java ME platform is a subset of the Java SE platform, having a small footprint, and is suitable for developing mobile and embedded applications. The Java Card platform allows development of embedded applications that have a very tiny memory footprint, targeting devices like smart cards. The upshot of this classification is that a Java program developed for one Java platform will not necessarily run under the JVM of another Java platform. The JVM must be compatible with the Java platform that was used to develop the application.

The API and the tools for developing and running Java applications are bundled together as the JDK. Starting with Java 11, JRE (Java Runtime Environment) is no longer available as a stand-alone bundle providing runtime support for execution of Java programs, but it continues to be a subset of the now modular JDK. As before, one needs to install the JDK to both develop and run Java programs. However, to deploy Java programs, the JDK tool jlink can be used to create a runtime image that includes the program code and the necessary runtime support to run the program—a topic that we will get to when we discuss modules.

We highly recommend installing the JDK for Java SE 17 depending on the hardware and operating system. Although newer versions of Java are released periodically, Java SE 17 is readily available as an LTS (long-term support) release, and is the subject of this book.

As of Java SE 17, Oracle is making the Oracle JDK available for free under the Oracle No-Fee Terms and Conditions (NFTC) license. Although subject to the conditions, it permits free use for all users.