IP Addresses
An IP address (Internet Protocol address) is a unique number that devices use in order to identify and communicate with each other on a computer network. They're called 'IP addresses' because they comply with the Internet Protocol standard (IP) - a set of rules that defines communication of data across a network.
Any device that is part of a network (such as computers, printers, Internet fax machines, and some telephones) must have its own unique address. An IP address can also be thought of as the equivalent of a street address or a phone number for a computer or other network device on the internet. Just as each street address and phone number uniquely identifies a building or telephone, an IP address can uniquely identify a specific computer or other network device on a network.
Generally, IP addresses aren't very people-friendly, so for most practical purposes on the Internet, they are converted into domain name such as 'aol.com' etc.
Finding out where someone has come from via their IP address is called an 'ip to hostname lookup' and involves translating an IP address into a domain name. An IP address, or a group (block) of addresses, can appear to be shared by multiple clients (users and their Internet devices).
A given web site, for example, might appear to 'share' an address with another unrelated site because both used shared web hosting. Different site visitors might share addresses because they actually connect to the Internet via their ISP which acts as an intermediary agent on behalf of its customers. In the latter case, the 'real' originating IP addresses is only available to the ISP company who often record this information to assist in tracing back complaints.
The best analogy would be telephone networks. One hundred people might dial outside lines via an internal telephone system but Caller Identification would only reveal a single number used by all 100. The only people who could trace the phone that placed a given call would be the internal system's administrators.