Designing Your IP Addressing Scheme

After you have received your network number, you can then plan how you will assign the host parts of the IP address.

Table 3-1 shows the division of the IP 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 IP address that are dedicated to the network part of the address, with each byte represented by xxx. "Host address" indicates the number of bytes 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 are dedicated to the host. The opposite is true for a class C network.

Table 3-1 Division of IP 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 IP address define whether the network is class A, B, or C and are always assigned by the InterNIC. 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 assigned to you.

Table 3-2 shows which bytes of the IP address are assigned to you and 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