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.
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