Most people today are familiar with the formal of addresses used on the Internet. A typical simple example might be:
All Internet addresses are instances of uniform resource identifiers (see URI) and follow the rules laid down (2005) in the standard RFC3986. The example above consists of two distinct parts, separated by a colon:
The scheme specifies what kind of resource is being sought at the specified address. Often the scheme name comes from the protocol to be used: “http” means that the message being sent will use the hypertext transfer protocol (HTTP) and is therefore seeking an HTTP server at the address. Similarly, the scheme “ftp;” indicates a message using the file transfer protocol (FTP) and seeking an FTP server. However, it is not always the case that the scheme specifies the resource: for example, the “mailto” scheme is used to specify an e-mail address, but e-mails are sent using the simple mail transfer protocol (SMTP).
The host is usually specified in plain-language form. It cannot be used directly but must first be translated into the equivalent IP address, which consists of a list of numbers. The Internet uses a domain name system, in which the plain-language address is treated as a hierarchy of component domains separated by full stops. The hierarchy is read from right to left, and the topmost level – in this case “com” – has to be one of the top-level domains defined by the Internet Corporation for Assigned Names and Numbers (ICANN).
In analyzing the address, each domain is submitted in turn to an appropriate domain name server (DNS), beginning with the top-level domain on the far right. The DNS returns information on where lo find the server for the next domain: when the end of the domain hierarchy is reached, an IP address will be returned.
This IP address can also be specified directly. Assuming that the IP address for “http://www.companyname.com” is 10.11.12.13,
means the same as “http://www.companyname.com”. This form of the address is only used in practice for special reasons.
The domain name “com” in the example is the commonest example of a generic top-level domain (gTLD), i.e. one with a name that indicates the type of site (in this case, a commercial company). A number of other generic domain names are approved by ICANN: for example “ac” for an academic institution, “gov” for a government organization, etc. Often, the top-level domain is a country domain indicating the country or region (“uk” for the United Kingdom, “us” for the United States, “eu” for the European Union, etc.). The generic name is then a second-level domain: for example, an academic institution in France might have the address:
or a company in the EU might be:
Note that “co” is used as a second-level name rather than “com”. Full lists of both generic and country approved domain names are given in the Appendix.