An IP address is the numeric address that is unique to your computer when on the Internet (or any other TCP/IP network for that matter). These numbers are hard to remember, so the domain nameing service (DNS) was invented that is essentially a large table mapping friendly names (such as www.theanswerbank.co.uk) to their respective IP address. The reverse process is possible using a DNS server, and in particular its 'ARP' tables. Given a supplied IP address, it will resolve it's friendly name. However! Since more than one friendly name can be mapped to any one IP address, obviously this would not work the other way round. As such, an ARP table holds only one record for each IP address, and this is usually the default name for any given machine (such as a web server that holds many websites in tandem).