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Sunday, August 28, 2011

CCNA - Understanding a class B network address

Class B Network Address

The designers of the Internet decided to create classes of networks based on network size.
A “class B” network address has its first 2 octets (2bytes or 16 bits) assigned to identify the network and the last 2 octets (16 bits) assigned to identify a host on that network.

If     N = Network bit
   H = Host bit

Class A: NNNNNNNN.HHHHHHHH.HHHHHHHH.HHHHHHHH
Class B: NNNNNNNN.NNNNNNNN.HHHHHHHH.HHHHHHHH
Class C: NNNNNNNN.NNNNNNNN.NNNNNNNN.HHHHHHHH

A bit can take on a two values (0 or 1), meaning;

Class A can have up to 2^8 = 256 networks and 2^24 = 16,777,216 hosts.
Class B can have up to 2^16 = 65,536 networks and 2^16 = 65,536 hosts.
Class C can have up to 2^24 = 16,777,216 networks and 2^8 = 256 hosts.

For the small number of networks possessing a very large number of nodes, they created the rank “Class A” network. At the other extreme is the “Class C” network, which is reserved for the numerous networks with a small number of nodes. The class distinction for networks between very large and very small is predictably called the “Class B” network.

In a “Class B” network, the first bit of the first byte must always be turned ON (set to 1) but the second bit must always be turned OFF (set to 0). If you turn the other 6 bits all OFF and then all ON, you will find the range for a Class B network:

10000000 = 128
10111111 = 191

As you can see, a Class B network is defined when the first byte is configured from 128 to 191.
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