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

Lab Activity Lab Exercise: Basic Subnetting This lab is to identify reasons to use a subnet mask and to distinguish between a default subnet mask and custom subnet mask.

    

Lab Activity Lab Exercise: Subnetting a Class A Network This lab is to analyze a Class A network address with the number of network bits specified in order to determine subnet mask, number of subnets, hosts per subnet, and information about specific subnets.

    

Lab Activity Lab Exercise: Subnetting a Class B Network This lab is to provide a subnetting scheme using a Class B network.

    

Lab Activity Lab Exercise: Subnetting a Class C Network This lab is to provide a subnetting scheme using a Class C network.

    

Web Links IP Addressing Subnetting Tutorial http://www.ralphb.net/IPSubnet/