Overview of Classless Addressing
Classless addressing (described in RFC 1519) abolishes the idea of traditional classes A, B. and C major nets and the notion of a subnet field. Subnets and major nets do not exist in a classless world; instead, there is only a network prefix and a host field. Figure I-7 describes the difference between classful and classless addressing.
Figure 1-7 Classful Versus Classless Addressing
|
Address |
Network field |
Subnet field |
Host field |
|
Mask |
ONES |
ZEROS | |
|
Classless Addressing: | |||
|
Address |
Prefix |
Host field | |
|
Mask |
ONES |
ZEROS | |
The length of the network prefix is determined by a prefix mask. The prefix mask is a contiguous series of ones that starts with the left-most bit (the most significant bit). Although the prefix mask looks like a subnet mask, it's important to realize that there is no subnet field.
An advantage of classless addressing is the capability to combine what were multiple class C addresses into a contiguous block of addresses called a supemet or classless interdomain routing (CIDR) block. Figure 1 -8 describes an address space in two ways: as four class C major nets (classful sense) and as one supernet (classless sense).
Figure 1 -8 An Address Space Written as Four Class C Major Nets and as One Supernet
Classful description
Major net 192.168.4.0
Major net 192.168.5.0
Major net 192.168.6.0
Classless description
192.168 4 0/22
Major net 192.168.7.0
Same address space
The number after the slash in the classless notation is the prefix length and indicates how many one bits are in the prefix mask. For example. 192.168.4.0/22 represents a prefix of 192.168.4.0 with a mask of 22 contiguous ones. The mask 122 is equivalent to 255.255.252.0 in dotted decimal notation.
With the prefix and the mask, you can determine the addresses covered by the supernet
Example 1 -2 Determining the Address Range Covered by 192 168.4.0/22
Prefix: 192.168.4.0 —► 192.168.0000 01 Mask: /22 >255.255.1111 -11
Address: 192.168.0000-0100.0000-0000 Through 192.168 0000-0111.1111-1111 CoT)
192.168.4.0 through 192.168.7.255
00.0000 00.0000
0000 0000
In a classless world, the address space depicted in Example 1-2 is one block and, if desired, may be deployed as one "subnet" supporting up to 1022 hosts (210. subtracting for the network prefix address itself and the all-ones pattern for the network prefix broadcast address). This demonstrates the power of classless addressing, if you had to use classful addressing, you would be stuck with four separate class C major nets the largest subnet you could make would be 254 hosts. (A class C major net with no subnetting yields 8 bits in the host field, which is 28 -2 = 254 hosts.)
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