The internet is based on the TCP/IP network protocol in which all of the computers connected to each other must have a unique IP address. Because it is bothersome to remember IP addresses (eg: 81.25.78.235) we can assign user friendly names to them. This is implemented by Domain Name System (DNS) which involves a hierarchical database of host names. The fully qualified domain name (FQDN) of a computer is comprised of the host name, the domain or subdomain the host belongs to and any domains above that in the hierarchy until the root (".") domain. An example being: www.sales.europe.domain.com. (the last full stop is the root domain). The .com bit is called the TOP-LEVEL domain which is broken down into different catagories depending on the organisation: .com is a commercial organisation, .net is networks (the backbone of the internet), .uk is the two letter country code for the UK. The root domain is usually dropped as it is more of a symbol rather than a DNS requirement.