TCP/IP---subnet addressing

All hosts are now required to support subnet addressing (RFC 950 [mogul and Postel 1985]). Not the I P address
As a simple network number and a host number, but the host number is divided into a subnet number and a host number.

The reason for this is because the Class A and Class B addresses allocate too much space for the host number, and the number of hosts that can be accommodated separately is
22 4-2 and 21 6-2. In fact, in a network people do not have so many hosts (various I p address format 1-5
Shown below). Because all 0 or 1 of the host number is invalid, we subtract the total by 2.

When i n T e R n i c obtains a certain type of I p network number, it is assigned by the local system administrator, he (or she)
Subnets, and how many bits are allocated to the subnet number and host number. For example, here is a Class B network address
(1 4 0. 2 5 2), in the remaining five bits, 8 bit is used for the subnet number, and 8 bit for the host number, as shown in format 3-5. Such
There are 2 5 4 subnets allowed, and each subnet can have 2 5 4 hosts.

TCP/IP---subnet addressing