Microsoft Windows graphic

IP addressing

Each TCP/IP host is identified by a logical IP address. This address is unique for each host that communicates by using TCP/IP. Each 32-bit IP address identifies a location of a host system on the network in the same way that a street address identifies a house on a city street.

Just as a street address has a standard two-part format (a street name and a house number), each IP address is separated internally into two parts—a network ID and a host ID:

Here is an example of a 32-bit IP address:

10000011 01101011 00010000 11001000

To make IP addressing easier, IP addresses are expressed in dotted decimal notation. The 32-bit IP address is segmented into four 8-bit octets. The octets are converted to decimal (base-10 numbering system) and separated by periods. Therefore, the previous IP address example is 131.107.16.200 when converted to dotted decimal notation.

For more information about dotted decimal notation and the conversion of numbers from binary to decimal, see Converting binary to decimal.

The following illustration shows a sample view of an IP address (131.107.16.200) as it is divided into network and host ID sections. The network ID portion (131.107) is indicated by the first two numbers of the IP address. The host ID portion (16.200) is indicated by the last two numbers of the IP address.

The IP address: network ID and host ID

Notes

IP address classes

The Internet community has defined five address classes. Class A, B, and C addresses are used for assignment to TCP/IP nodes.

The class of address defines which bits are used for the network and host ID parts of each address. The address class also defines how many networks and hosts per network can be supported.

The following table uses w.x.y.z to designate the four octet values in any given IP address. The table is used to show:

Class Value of w Network ID Host ID Number of networks Number of hosts per network
A 1–126 w x.y.z 126 16,777,214
B 128–191 w.x y.z 16,384 65,534
C 192–223 w.x.y z 2,097,152 254
D 224–239 Reserved for multicast addressing N/A N/A N/A
E 240–254 Reserved for experimental use N/A N/A N/A