21 Sep 06
An IP address is a 32 bit binary number usually represented as 4 decimal values, each representing 8 bits, in the range 0 to 255 (known as octets) separated by decimal points. This is known as "dotted decimal" notation. Example: 126.96.36.199 It is sometimes useful to view the values in their binary form. 140 .179 .220 .200 10001100.10110011.11011100.11001000 Every IP address consists of two parts, one identifying the network and one identifying the node. The Class of the address and the subnet mask determine which part belongs to the network address and which part belongs to the node address. Address Classes There are 5 different address classes. You can determine which class any IP address is in by examining the first 4 bits of the IP address. * Class A addresses begin with 0xxx, or 1 to 126 decimal. * Class B addresses begin with 10xx, or 128 to 191 decimal. * Class C addresses begin with 110x, or 192 to 223 decimal. * Class D addresses begin with 1110, or 224 to 239 decimal. * Class E addresses begin with 1111, or 240 to 254 decimal. Addresses beginning with 01111111, or 127 decimal, are reserved for loopback and for internal testing on a local machine. [You can test this: you should always be able to ping 127.0.0.1, which points to yourself] Class D addresses are reserved for multicasting. Class E addresses are reserved for future use. They should not be used for host addresses. Now we can see how the Class determines, by default, which part of the IP address belongs to the network (N) and which part belongs to the node (n). * Class A -- NNNNNNNN.nnnnnnnn.nnnnnnnn.nnnnnnnn * Class B -- NNNNNNNN.NNNNNNNN.nnnnnnnn.nnnnnnnn * Class C -- NNNNNNNN.NNNNNNNN.NNNNNNNN.nnnnnnnn In the example, 188.8.131.52 is a Class B address so by default the Network part of the address (also known as the Network Address) is defined by the first two octets (140.179.x.x) and the node part is defined by the last 2 octets (x.x.220.200). In order to specify the network address for a given IP address, the node section is set to all "0"s. In our example, 184.108.40.206 specifies the network address for 220.127.116.11. When the node section is set to all "1"s, it specifies a broadcast that is sent to all hosts on the network. 18.104.22.168 specifies the example broadcast address. Note that this is true regardless of the length of the node section. More logic than maths, although maths obviously plays apart.