An IP address is a 32-bit number that identifies each sender or receiver of information that is sent in packet across the Internet. Consisting of network and node portions, the address is represented in standard decimal notation (for example, 123.45.6.7).

When you request an HTML page or send e-mail, the Internet Protocol part of TCP/IP includes your IP address in the message. It then sends it to the IP address that is obtained by looking up the domain name in the URL you requested or in the e-mail address you're sending a message to. At the other end, the recipient can see the IP address of the Web page requester or the e-mail sender and can respond by sending another message using the IP address it received.

An IP address has two parts: the identifier of a particular network on the Internet and an identifier of the particular device (which can be a server or a workstation) within that network. On the Internet itself---that is, between the routers that move packets from one point to another along the route---only the network part of the address is looked at.