An IP address (Internet Protocol address) is a unique number that identifies a computer during communication across a network. The IP address is theoretically required to be unique for all devices on the network.
However, not all computers on the Internet have been assigned their own unique IP address, because there are not enough addresses to go around. This lack of addresses has been remedied by several kludges. One such kludge is called NAT (Network Address Translation), whereby all computers on one subnet share a "public" IP address for external communication while maintaining a "private" address for internal communication. NAT has been effective in prolonging the useable lifespan of the now twenty-year-old IP address protocol. A new version of the IP protocol, IPv6, is designed to replace the current protocol, but has not yet been widely implemented. The new version provides a vastly larger address space and simpler autoconfiguration.
An IP address assigned to a computer can be used to find the identity of the owner of that computer. IP addresses are a particular anonymity concern for MAPs, because they are revealed to the recipients of e-mail and to web sites visited. A particular threat is IP harvesting, or the systematic collection of IP addresses belonging to, for instance, MAPs.
Many MAPs choose to use a proxy server or VPN to hide their IP address. Unfortunately, hostile proxy server operators may be able to identify which websites an IP address connects to, which has persuaded some MAPs to use onion routers such as Tor.