An internet adress is a 32-bit number used to uniquely identify each host attached to an internet. We write addresses as a sequence of four eight-bit decimal numbers, separated by periods ("dot format").
TCP/IP implementations consider addresses as a two-part value: a network portion and a host portion.
There are three clases of internet networks defined for TCP/IP internets. The table below shows how the network portion of a dot format address of X.X.X.X takes up from one to three of the leading X's.
Class Network Portion Host Portion Numberof Hosts Sample IP Address
A 0-127 X.X.X 16,777,216 10.55.110.222
B 128-191.X X.X 65,536 128.123.90.2
C 192-255.X.X X 256 192.5.10.69
X is a value between 0 and 255.