Uniform Resource Identifier (URI) is an addressing technology from which a Uniform Resource Locator (URL) is created. URLs that use the http:// and ftp:// protocols are types of URIs.

Routers use reference tables to decode the URIs they receive in packet headers and route the packets to the correct destination server or domain name server. Domain Name System (DNS) servers use the same method to decode the URI and send the packets to their final destination. This destination can be a web site, an FTP site, or any other available service. When you register a domain name, a plain-language scheme, with a registration service, they cross-reference your numeric URI with the URI you have registered. When someone enters your registered domain name in a browser address field to access your web site, the routers and domain name servers have the information they need to route that traffic to and from your site.

The complete URI describes the mechanism used to access the resource, the computer name where the resource is located and the name of the resource (a file name) on the computer. The letters to the left of the colon specify the access scheme (protocol). The format of the alphanumeric information to the right of the colon depends on the scheme and contains the host address and the path to the file you want to access. The port number that serves the protocol follows a second colon to the right of the host name. If the port number is not inserted, the access protocol attempts a connection using the default port for the specified protocol. For example, http (Hyper Text Transfer Protocol) service uses the default port 80. Therefore, http://hostname:80/filename and http://hostname/filename are valid.