To keep all of the machines on the Internet straight, each machine is assigned a unique address called an IP address. IP stands for Internet protocol, and these addresses are 32-bit numbers normally expressed as four "octets" in a "dotted decimal number." A typical IP address looks like this:

          216.183.103.150
                  

The four numbers in an IP address are called octets because they can have values between 0 and 255 (2⁸ possibilities per octet).

Every machine on the Internet has its own IP address. A server has a static IP address that does not change very often. A home machine that is dialing up through a modem often has an IP address that is assigned by the ISP when you dial in. That IP address is unique for your session and may be different the next time you dial in. In this way, an ISP only needs one IP address for each modem it supports, rather than for every customer.

If you are working on a Windows machine, you can view your current IP address with the command WINIPCFG.EXE (IPCONFIG.EXE for Windows 2000/XP). On a UNIX machine, type nslookup along with a machine name (such as "nslookup www.mysite.com") to display the IP address of the machine (use the command hostname to learn the name of your machine).

For more information on IP addresses, see IANA.

As far as the Internet's machines are concerned, an IP address is all that you need to talk to a server. For example, you can type in your browser the URL http://217.204.37.8 and you will arrive at the machine that contains the Web server for webconcerns. Domain names are strictly a human convenience.