IP address

IP address is short for Internet Protocol (IP) address. An IP address is an identifier for a computer or device on a TCP/IP network. Networks using the TCP/IP protocol route messages based on the IP address of the destination. Contrast with IP, which specifies the format of packets, also called datagrams, and the addressing scheme.

The format of an IP address is a 32-bit numeric address written as four numbers separated by periods. Each number can be zero to 255. For example, 1.160.10.240 could be an IP address.

Within an isolated network, you can assign IP addresses at random as long as each one is unique. However, connecting a private network to the Internet requires using registered IP addresses (called Internet addresses) to avoid duplicates.

Static Versus Dynamic IP Addresses

An IP address can be static or dynamic. A static IP address will never change and it is a permanent Internet address. A dynamic IP address is a temporary address that is assigned each time a computer or device accesses the Internet.

The four numbers in an IP address are used in different ways to identify a particular network and a host on that network. Four regional Internet registries — ARIN, RIPE NCC, LACNIC and APNIC– assign Internet addresses from the following three classes:

Class A – supports 16 million hosts on each of 126 networks
Class B – supports 65,000 hosts on each of 16,000 networks
Class C – supports 254 hosts on each of 2 million networks

The Two Parts of an IP Address

An IP address consists of two parts, one identifying the network and one identifying the node, or host.

The Class of the address determines which part belongs to the network address and which part belongs to the node address. All nodes on a given network share the same network prefix but must have a unique host number.

Class A Network
In a Class A Network binary address start with 0, therefore the decimal number can be anywhere from 1 to 126. The first 8 bits (the first octet) identify the network and the remaining 24 bits indicate the host within the network. An example of a Class A IP address is 102.168.212.226, where “102” identifies the network and “168.212.226” identifies the host on that network.

Class B Network
In a Class B Network, binary addresses start with 10, therefore the decimal number can be anywhere from 128 to 191. The number 127 is reserved for loopback and is used for internal testing on the local machine. The first 16 bits (the first two octets) identify the network and the remaining 16 bits indicate the host within the network. An example of a Class B IP address is 168.212.226.204 where “168.212” identifies the network and “226.204” identifies the host on that network.

Class C Network
Binary addresses start with 110, therefore the decimal number can be anywhere from 192 to 223. The first 24 bits (the first three octets) identify the network and the remaining 8 bits indicate the host within the network. An example of a Class C IP address is 200.168.212.226 where “200.168.212” identifies the network and “226” identifies the host on that network.

Class D Network
In a Class D Network, binary addresses start with 1110, therefore the decimal number can be anywhere from 224 to 239. Class D networks are used to support multicasting.

Class E Network
In a Class E Network, binary addresses start with 1111, therefore the decimal number can be anywhere from 240 to 255. Class E networks are used for experimentation. They have never been documented or utilized in a standard way.

The number of unassigned Internet addresses is running out, so a new classless scheme called CIDR is gradually replacing the system based on classes A, B, and C and is tied to adoption of IPv6. In IPv6 the IP address size is increased from 32 bits to 128 bits.
From Webpedia