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Subnetting Class A and B networks

Subnetting Class A and B networks
10.3.5 This page will describe the process used to subnet Class A, B, and C networks.


The Class A and B subnetting procedure is identical to the process for Class C, except there may be significantly more bits involved. The available bits for assignment to the subnet field in a Class A address is 22 bits while a Class B address has 14 bits.

Assigning 12 bits of a Class B address to the subnet field creates a subnet mask of 255.255.255.240 or /28. All eight bits were assigned in the third octet resulting in 255, the total value of all eight bits. Four bits were assigned in the fourth octet resulting in 240. Recall that the slash mask is the sum total of all bits assigned to the subnet field plus the fixed network bits.

Assigning 20 bits of a Class A address to the subnet field creates a subnet mask of 255.255.255.240 or /28. All eight bits of the second and third octets were assigned to the subnet field and four bits from the fourth octet.

In this situation, it is apparent that the subnet mask for the Class A and Class B addresses appear identical. Unless the mask is related to a network address it is not possible to decipher how many bits were assigned to the subnet field.

Whichever class of address needs to be subnetted, the following rules are the same:

Total subnets = 2 to the power of the bits borrowed
Total hosts = 2 to the power of the bits remaining
Usable subnets = 2 to the power of the bits borrowed minus 2
Usable hosts = 2 to the power of the bits remaining minus 2

The next page will discuss logical ANDing.

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