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From Wikipedia, the free encyclopedia

A hostname is a label that is assigned to a device connected to a computer network and that is used to identify the device in various forms of electronic communication such as the World Wide Web, e-mail or Usenet. Hostnames may be simple names consisting of a single word of phrase, or they may include the name of a Domain Name System (DNS) domain at the end, that is separated from the host specific label by a full stop (dot). In the latter form, a hostname is also called a domain name. If the domain name is completely specified including a top-level domain of the Internet, the hostname is said to be a fully qualified domain name (FQDN).

Hostnames that include DNS domains are often stored in the Domain Name System together with IP addresses of the host they represent for the purpose of mapping the hostname to an address, or the reverse process.



Hostnames are used by various naming systems, NIS, DNS, SMB, etc., and so the meaning of the word hostname will vary according to the naming system in question, which in turn varies by type of network. A hostname meaningful to a Microsoft NetBIOS workgroup may be an invalid Internet hostname. When presented with a hostname and no context, it is usually safe to assume that the network is the Internet and DNS is the hostname's naming system.

Host names are typically used in an administrative capacity and may appear in computer browser lists, active directory lists, IP address to hostname resolutions, email headers, etc. They are human-readable nicknames, which ultimately correspond to unique network hardware MAC addresses. In some cases the host name may contain embedded domain names and/or locations, non-dotted IP addresses, etc.

On a simple local area network, a hostname is usually a single word: for instance, an organization's CVS server might be named "cvs" or "server-1".

Internet hostnames

On the Internet, a hostname is a domain name assigned to a host computer. This is usually a combination of the host's local name with its parent domain's name. For example, "" consists of a local hostname ("en") and the domain name "". This kind of hostname is translated into an IP address via the local hosts file, or the Domain Name System (DNS) resolver. It is possible for a single host computer to have several hostnames; but generally the operating system of the host prefers to have one hostname that the host uses for itself.

Any domain name can also be a hostname, as long as the restrictions mentioned below are followed. So, for example, both "" and "" are hostnames because they both have IP addresses assigned to them. The domain name "" is not a hostname since it does not have an IP address, but "" is a hostname. A hostname may be a domain name, if it is properly organized into the domain name system. A domain name may be a hostname if it has been assigned to an Internet host and associated with the host's IP address.


Restrictions on valid host names

Hostnames are composed of series of labels concatenated with dots, as are all domain names[1]. For example, "" is a hostname. Each label must be between 1 and 63 characters long, and the entire hostname has a maximum of 255 characters.

The Internet standards (Request for Comments) for protocols mandate that component hostname labels may contain only the ASCII letters 'a' through 'z' (in a case-insensitive manner), the digits '0' through '9', and the hyphen. The original specification of hostnames in RFC 952, mandated that labels could not start with a digit or with a hyphen, and must not end with a hyphen. However, a subsequent specification (RFC 1123) permitted hostname labels to start with digits. No other symbols, punctuation characters, or blank spaces are permitted.

While a hostname may not contain other characters, such as the underscore character (_), names entered into the DNS, may contain the underscore. Systems such as DomainKeys and service records use the underscore as a means to assure that their special character is not confused with hostnames. For example, specifies a service pointer for an SCTP capable webserver host (www) in the domain

A notable example of non-compliance with this specification, Windows systems often use underscores in hostnames. Since some systems will reject invalid hostnames while others will not, the use of invalid hostname characters may cause subtle problems in systems that connect to standards-based services. For example, RFC-compliant mail servers will refuse to deliver mail for MS Windows computers with names containing underscores.

So, the hostname "" is made up of the DNS labels "en", "wikipedia" and "org". Labels such as "2600" and "3com" can be used in hostnames, but "-hi-" and "*hi*" are invalid.

A hostname is considered to be a fully qualified domain name (FQDN) if all the labels up to and including the top-level domain name (TLD) are specified. The hostname "" terminates with the top-level domain "org" and is thus fully-qualified. Depending on the system, an unqualified hostname such as "compsci" or "wikipedia" may be combined with default domain names in order to determine the fully qualified domain name. So, a student at Harvard may be able to send mail to "joe@compsci" and have it automatically qualified by the mail systems so that it is sent to "".

General guidelines on choosing good hostnames are outlined in RFC 1178.

See also


External links

  • RFC 952 - "DoD Internet host table specification."
  • RFC 1034 - "DOMAIN NAMES - CONCEPTS AND FACILITIES" (In particular, section 3.5)
  • RFC 1035 - "DOMAIN NAMES - IMPLEMENTATION AND SPECIFICATION" (In particular, section 2.3.1)
  • RFC 1123 - "Requirements for Internet Hosts - Application and Support."
  • RFC 1178 - "Choosing a Name for Your Computer"
  • RFC 3696 - "Application Techniques for Checking and Transformation of Names"

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

Simple English

A hostname (also known as a sitename) is the unique name by which a network device (which may be a computer, file server, network storage device, copier, cable modem, etc.) is defined and known on a network. The hostname is used to identify a particular host in various electronic communication networks such as the World Wide Web, e-mail or Usenet.

On the Internet, the terms "hostname" and "domain name" are often misused interchangeably, but there are small technical differences between them.

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