Each computer on the Internet has a unique numerical address, called an Internet Protocol (IP) address, used to route packets to it across the Internet.

Just as your postal address enables the postal system to send mail to your house from anywhere around the world, your computer's IP address gives the Internet routing protocols the unique information they need to route packets of information to your desktop from anywhere across the Internet. If a machine needs to contact another by a domain name, it first looks up the corresponding IP address with the domain name service. The IP address is the geographical descriptor of the virtual world, and the addresses of both source and destination systems are stored in the header of every packet that flows across the Internet.

An IP address is made up of four bytes of information (totaling 32 bits) expressed as four numbers between 0 and 255 shown separated by periods. For example, your computer's IP address might be 238.17.159.4, which is shown below in human-readable decimal form and in the binary form used on the Internet.

Each of the four numbers uses eight bits of storage, and so can represent any of the 256 numbers in the range between zero (binary 00000000) and 255 (binary 11111111). Therefore, there are more than 4 billion possible different IP addresses in all:

4,294,967,296   =   256 * 256 * 256 * 256

Allocations. The Internet Assigned Numbers Authority manages the allocation of IP addresses to different organizations in various sized blocks. The IANA IP Address Service page provides a focal point for this world wide IP address management. An official list of the allocations of IP address blocks can be found at the Internet Protocol Address Space site, and related information can be found at the IP Index Encyclopedia.

Most of the address blocks have been allocated to research, education, government, corporations, and Internet Service Providers, who in turn assign them to the individual computers under their control. A few addresses are reserved for future or special use. The historical top-level allocations of these blocks of IP addresses are described in Request For Comments 1466.