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A hostname (occasionally also, a sitename) is the unique
name by which a network-attached device (which could consist of a
computer, file server, network storage device, fax machine, copier,
cable modem, etc.) is known on a network. The hostname is used to
identify a particular host in various forms of electronic communication
such as the World Wide Web, e-mail or Usenet.
On the Internet, the terms "hostname" and "domain name" are often used
interchangeably, but there are subtle technical differences between them.
Hostnames are used by various naming systems,
NIS, DNS, SMB, etc., and so the meaning of the word hostname will vary depending
on 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 nick-names, 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, a hostname is a
name assigned to the host. This is usually a combination of the host's local
name with its parent domain's name. For example, "en.wikipedia.org" consists of
a hostname ("en") and the domain name "wikipedia.org". 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 to have
several hostnames; but generally the
operating system of the host prefers to have one hostname that the host uses
Any domain name can also be hostname, as long as the restrictions mentioned
below are followed. So, for example, both "en.wikimedia.org" and "wikimedia.org"
are hostnames because they both have IP
addresses assigned to them. The domain name "pmtpa.wikimedia.org" is not a
hostname since it does not have an IP address, but "rr.pmtpa.wikimedia.org" is a
hostname. All hostnames are domain names, but not all domain names are
Restrictions on valid host names
Hostnames, like all domain names,
are made up of a series of "labels", with each label being separated by a dot.
Each label must be between 1 and 63 characters long, and there is a maximum of
255 characters when all labels are combined.
Unlike domain names, hostname labels can only be made up of the
ASCII letters 'a'
through 'z' (case-insensitive), the digits '0' through '9', and the hyphen.
Labels can not start nor end with a hyphen. Special characters other than the
hyphen (and the dot between labels) are not allowed, although they are sometimes
used anyway. Underscore characters are commonly used by Windows systems but
RFC 952 they are not allowed and several systems, such as
DomainKeys and the SRV record deliberately use the underscore to make sure their special domain
names are not confused with a hostname. Since some systems will check to make
sure that hostnames contain only valid characters and others do not, the use of
the invalid characters such as the underscore has caused many subtle problems in
systems that connect to the wider world.
So, the hostname "en.wikipedia.org" 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. 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 sent to
Choosing host names
General guidelines on choosing a good hostnames are outlined in
RFC 1178. The folklore interest of hostnames stems from the creativity and
humour they often display. Interpreting a sitename is not unlike interpreting a
vanity licence plate; one has to mentally unpack it, allowing for mono-case and
length restrictions and the lack of whitespace. Hacker tradition deprecates
dull, institutional-sounding names in favour of punchy, humorous, and clever
coinages (except that it is considered appropriate for the official public
gateway machine of an organisation to bear the organisation's name or acronym).
Mythological references, cartoon characters, animal names, and allusions to
sci-fi or fantasy literature are probably the most popular sources for sitenames
(in roughly descending order). The obligatory comment is
Harris's lament: "All the good ones are taken!"
It is often possible to guess a hostname for a particular institution. This
is useful if you want to know if they operate network services like anonymous
FTP, World-Wide Web or finger. First try the institution's name or obvious
abbreviations thereof, with the appropriate domain appended, e.g. "mit.edu". If
this fails, prepend "ftp." or "www." as appropriate, e.g. "www.data-io.com". You
can use the ping command as a
quick way to test whether a hostname is valid.
Notes and references
RFC 952 - "DoD Internet host table specification."
RFC 1034 - "DOMAIN NAMES - CONCEPTS AND FACILITIES" (In particular,
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
This article was originally based on
material from the
Free On-line Dictionary of Computing, which is
licensed under the
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