IP Addresses

Present common carrier communications networks…. use links and concepts originally designed for another purpose — voice…. we might wish to consider an alternative approach — a standardized message block… composed of perhaps 1024 bits. Most of the message block would be reserved for whatever type data is to be transmitted, while the remainder would contain housekeeping information such as error detection and routing data.

Paul Baran, On Distributed Communications, Volume I, 1964.

What is an IP address? 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, 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.

You can find your IP address on a Windows computer by opening an MSDOS or Command window and typing one of “winipcfg” or “ipconfig”. You can find your IP address on a Mac computer by checking your Network control panel. No matter what electronic device you are using and where you are, if you are connected to the web you can visit the following sites to dynamically find your IP address in real time:

As described in the pages on confidentiality and privacy, Internet sites can and do track your IP address and other information. If you want to block or disguise your IP address, you can use an anonymizer.

The sections below provide more information about the IP address format, allocations, lookup databases, and references to more information.

Format. 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, which is shown below in human-readable decimal form and in the binary form used on the Internet.

Example IP Address


238 . 17 . 159 . 4


11101110   00010001   10011111   00000100

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 Allocations 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.

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.

Lookup databases. You can find out more information on any particular IP number by searching one of the following databases. Since each address is generally kept in only one database, you might have to try more than one to find the database that has the information on any particular address:

You can also search the web for information about an IP address; for example, is there any information about address

Resources. The following resources provide more information:

  • RFC 796; J. Postel; Address Mappings; Sep 1981.
  • RFC 1518; An Architecture for IP Address Allocation with CIDR; Y. Rekhter, T. Li; September 1993.
  • RFC 1918; Address Allocation for Private Internets; Y. Rekhter, B. Moskowitz, D. Karrenberg, G. J. de Groot, E. Lear; February 1996.
  • RFC 2050; Internet Registry IP Allocation Guidelines; K. Hubbard, M. Kosters, D. Conrad, D. Karrenberg, J. Postel; November 1996.