Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 42
Class B Address

Okay, the answer is, 50 percent of the available address space is defined by Class A. How? Change the address Class bits to binary. Since the address is defined by the first bit alone and the next 31 bits are disregarded, it represents 50 percent of the available bits for address assignment (for those scratching their heads, it is 2ⁿ31 bits, which is 50 percent of the address space). Don’t think in a class–oriented environment. I simply asked how much of the address space can be defined by using 1 bit. This will become more apparent in the classless routing section.

Class B addresses take the form <network number.network number.host.host>, for bytes 0, 1, 2, and 3. This is the most requested class of address and is the easiest to assign subnets to. Class B addresses use the first 2 bytes of the 4 bytes for the network number and the last two fields for the host number. It is identified by the first 2 bits of the first byte. If the first bit is a 1, then the algorithm checks the second bit. If the second bit is a 0, this will identify a Class B address.

This allows for 16,384 network numbers (10111111.11111111.host.host or (2ⁿ14), with each network number capable of supporting 65,534 (2ⁿ16 –2) hosts (net.net.11111111.111 11110). Wait, there are 16 bits in the first two fields, this should allow for 65,535 networks. Since Class B reserves the first 2 bits to identify the class type (in binary, a 10xxxxxx in the first field), there are limited address numbers that may be used in the first field (valid range becomes 2ⁿ14). This translates to 128–191 (in decimal) as the allowable network numbers in the first field. Since the first field identifies the class, the second field is free to use all 8 bits, and can range from 0 to 255. The total range for network numbers for Class B addresses is 128 to 191 (in the first field), 0 to 255 (in the second field), and xxx.xxx (x represents the host ID) in the third and fourth fields. This is the most popular class of addresses.

It provides the largest range of addressing possibilities. However, unless companies have handed in their Class B addresses, this class is exhausted and they are no longer given out.

Okay, let’s try again. How much of the available address space is defined by Class B’s reserved first 2 bits? The answer is on the next page.

Class B Address