Designing Your IPv4 Addressing Scheme

After you receive your network number, you can then plan how to assign the host parts of the IPv4 address.

The following table shows the division of the IPv4 address space into network and host address spaces. For each class, “Range” specifies the range of decimal values for the first byte of the network number. “Network Address” indicates the number of bytes of the IPv4 address that are dedicated to the network part of the address. Each byte is represented by xxx. “Host Address” indicates the number of bytes that are dedicated to the host part of the address. For example, in a class A network address, the first byte is dedicated to the network, and the last three bytes are dedicated to the host. The opposite designation is true for a class C network.

Table 3–1 Division of IPv4 Address Space

Class	Range	Network Address	Host Address
A	0–127	xxx	xxx.xxx.xxx
B	128–191	xxx.xxx	xxx.xxx
C	192–223	xxx.xxx.xxx	xxx

The numbers in the first byte of the IPv4 address define whether the network is class A, B, or C. InterNIC assigns the numbers. The remaining three bytes have a range from 0–255. The numbers 0 and 255 are reserved. You can assign the numbers 1–254 to each byte, depending on the network number that is assigned to you.

The following table shows which bytes of the IPv4 address are assigned to you. The following table also shows the range of numbers within each byte that are available for you to assign to your hosts.

Table 3–2 Range of Available Numbers

Network Class	Byte 1 Range	Byte 2 Range	Byte 3 Range	Byte 4 Range
A	0–127	1–254	1–254	1–254
B	128–191	Preassigned by Internet	1–254	1–254
C	192–223	Preassigned by Internet	Preassigned by Internet	1–254