Every 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 (for example, example.com or example.com.ru), 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. For example, www.example.com.ru translates to 126.96.36.199. Another example, www.example.com translates to 188.8.131.52. Domain example.com resolve to a server managed by Internet Corporation for Assigned Names and Numbers (ICANN).
The original designers of TCP/IP defined an IP address as a 32-bit number and this system, known as Internet Protocol Version 4 or IPv4, is still in use today. However, due to the enormous growth of the Internet and the resulting depletion of available addresses, a new addressing system (IPv6), using 128 bits for the address, was developed in 1995 and last standardized by RFC 2460 in 1998. Although IP addresses are stored as binary numbers, they are usually displayed in human-readable notations, such as 184.108.40.206 (for IPv4), and 2001:db8:0:1234:0:567:1:1 (for IPv6).
The Internet Protocol also has the task of routing data packets between networks, and IP addresses specify the locations of the source and destination nodes in the topology of the routing system. For this purpose, some of the bits in an IP address are used to designate a subnetwork. The number of these bits is indicated in CIDR notation, appended to the IP address, e.g., 220.127.116.11/24.
Most of the address blocks have been allocated to research, education, government, corporations, and Internet Service Providers (ISP), 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.
If you connect to the Internet over a phone line, then your IP address is probably assigned dynamically by your Internet service provider from an available pool of addresses each time you log on. If your computer is permanently connected to an Internet network, such as at the office or on a high speed home connection, then your IP address could be permanently assigned, or could be reassigned each time you reboot your computer.