Editing Subdomain

{{Short description|DNS domains that are part of others}}
{{For|a protein domain inside of a larger domain|Protein domain}}
{{Use American English|date = February 2019}}

In the [[Domain Name System]] (DNS) hierarchy, a '''subdomain''' is a [[domain name|domain]] that is a part of another (main) domain.<ref name="RFC 1034 section 3.1">{{cite IETF |rfc=1034 |title=Domain names - concepts and facilities |section=3.1 |sectionname=Name space specifications and terminology |author=P. Mockapetris |author-link=Paul Mockapetris |date=November 1987 |publisher=[[Internet Engineering Task Force|IETF]] |access-date=2008-08-03}}</ref> For example, if a domain offered an online store as part of their website it might use the subdomain.

== Overview ==
The [[Domain Name System]] (DNS) has a [[tree structure]] or hierarchy, which includes nodes on the tree being a [[domain name]]. A subdomain is a domain that is part of a larger domain. Each label may contain from 0 to 63 [[octet (computing)|octets]].<ref name="rfc1034">RFC 1034, ''Domain Names - Concepts and Facilities'', P. Mockapetris (Nov 1987)</ref>  The full domain name may not exceed a total length of 253 ASCII characters in its textual representation.<ref name="rfc1035">RFC 1035, ''Domain names--Implementation and specification'', P. Mockapetris (Nov 1987)</ref>

Subdomains are defined by editing the DNS zone file pertaining to the parent domain. However, there is an ongoing debate over the use of the term "subdomain" when referring to names which map to the [[List of DNS record types#A|Address record]] A (host) and various other types of zone records which may map to any public [[IP address]] destination and any type of server. Network Operations teams insist that it is inappropriate to use the term "subdomain" to refer to any mapping other than that provided by zone [[Authoritative name server|NS]] (name server) records and any server-destination other than that.

According to RFC 1034, ''"a domain is a subdomain of another domain if it is contained within that domain"''.  Based on that definition, a host cannot be a subdomain, only a domain can be a subdomain.  A subdomain will also have a separate zone file with a SOA record (Start of Authority).

Most [[domain name registry|domain registries]] only allocate a two-level domain name. Hosting services typically provide DNS Servers to resolve subdomains within that master domain.

[[File:Subdomain-en.svg|thumb|300px|Example of subdomain]]

A fully qualified domain name consists of multiple parts. For example, take the English Wikipedia domain <code><nowiki>en.wikipedia.org</nowiki></code>.
The <code>en</code> is a subdomain of <code>wikipedia.org</code>.
Although <code>wikipedia.org</code> is usually considered to be the [[domain name]], <code>wikipedia</code> is actually a sub-domain of the <code>org</code> [[Top-level domain|TLD]] (top level domain). Any fully qualified domain name can be a host or a subdomain.

A domain name that does not include any subdomains is known as an ''apex domain'', ''root domain'', or ''bare domain''.<ref>{{cite web|url=https://docs.github.com/en/pages/configuring-a-custom-domain-for-your-github-pages-site/about-custom-domains-and-github-pages#using-an-apex-domain-for-your-github-pages-site|title=About custom domains and GitHub Pages § Using an apex domain for your GitHub Pages site|website=GitHub Docs|access-date=2021-04-09|archive-date=2021-08-08|archive-url=https://web.archive.org/web/20210808072525/https://docs.github.com/en/pages/configuring-a-custom-domain-for-your-github-pages-site/about-custom-domains-and-github-pages#using-an-apex-domain-for-your-github-pages-site|url-status=live}}</ref> For example, <code>wikipedia.org</code> is the apex domain of Wikipedia, which redirects to the subdomain <code>www.wikipedia.org</code>.

To discover more subdomains associated with a domain, you can utilize a variety of methods and tools. Automated tools like Amass<ref>{{Citation |title=owasp-amass/amass |date=2024-10-27 |url=https://github.com/owasp-amass/amass |access-date=2024-10-27 |publisher=OWASP Amass Project}}</ref> and Subfinder <ref>{{Citation |title=projectdiscovery/subfinder |date=2024-10-27 |url=https://github.com/projectdiscovery/subfinder |access-date=2024-10-27 |publisher=ProjectDiscovery}}</ref> leverage open-source intelligence and SSL certificate data<ref>{{Cite web |title=crt.sh {{!}} Certificate Search |url=https://crt.sh/ |access-date=2024-10-27 |website=crt.sh}}</ref> to quickly uncover subdomains. Google Dorking, using the "site:" operator, allows for manual searches of indexed subdomains, while brute force techniques systematically query DNS servers with potential names. Passive DNS reconnaissance through APIs from services like SecurityTrails & Subdomain Center<ref>{{Cite web |title=The World's Fastest Growing Subdomain & Shadow IT Database |url=https://www.subdomain.center/ |access-date=2024-10-27 |website=subdomain.center |language=en}}</ref> can reveal historical data without direct queries. Additionally, community resources such as GitHub and Pastebin may contain publicly available lists of subdomains. Combining these approaches will enhance your ability to effectively identify hidden or overlooked subdomains for security assessments or research purposes.<ref>{{Cite web |last=TheTechromancer |title=Subdomain Enumeration Tool Face-off - 2023 Edition |url=https://blog.blacklanternsecurity.com/p/subdomain-enumeration-tool-face-off-4e5 |access-date=2024-10-27 |website=blog.blacklanternsecurity.com |language=en}}</ref>

== Subdomain usage ==
{{refimprove section|date=March 2023}}

Subdomains are often used by internet service providers supplying web services. They allocate one (or more) subdomains to their clients who do not have their own domain name. This allows independent administration by the clients over their subdomain.

Subdomains are also used by organizations that wish to assign a unique name to a particular department, function, or service related to the organization.  For example, a university might assign "cs" to the computer science department, such that a number of hosts could be used inside that subdomain, such as <code>www.cs.example.edu</code>.<ref>{{Cite web |title=What Is A Subdomain? Everything You Need To Know – Forbes Advisor |author1=Shweta |author-first2=Kelly |author-last2=Main |others=Reviewed by Rob Watts |url=https://www.forbes.com/advisor/business/what-is-a-subdomain/#when_to_use_a_subdomain_section |access-date=2023-03-31 |website=www.forbes.com |archive-date=2023-03-31 |archive-url=https://web.archive.org/web/20230331140947/https://www.forbes.com/advisor/business/what-is-a-subdomain/#when_to_use_a_subdomain_section |url-status=live }}</ref>

There are some widely recognized subdomains such as WWW and [[FTP]]. This allows for a structure where the domain contains administrative directories and files including the FTP directories and webpages. The FTP subdomain could contain logs and the web page directories, while the WWW subdomain contains the  directories for the webpages. Independent authentication for each domain provides access control over the various levels of the domain.

== Uses ==
===United Kingdom===
In the [[United Kingdom]], the second-level domain names are standard and branch off from the [[top-level domain]]. For example:

* .ac.uk: [[academic]] ([[tertiary education]], [[further education colleges]] and [[research institute|research establishments]]) and [[learned society|learned societies]]
* .co.uk: general use (usually [[Commercial enterprise|commercial]])
* .gov.uk: [[government]] (central and local)
* .judiciary.uk: [[court]]s (to be introduced in the near future)<ref>{{cite web |title=UK court systems set to adopt judiciary.uk domain names |url=https://www.bbc.co.uk/news/technology-15860533 |publisher=[[BBC News]] |date=23 November 2011 |access-date=26 February 2014 |archive-date=4 July 2018 |archive-url=https://web.archive.org/web/20180704235839/https://www.bbc.co.uk/news/technology-15860533 |url-status=live }}</ref>
* .ltd.uk: [[limited company|limited companies]]
* .me.uk: general use (usually [[person]]al)
* .mod.uk: [[Ministry of Defence (United Kingdom)|Ministry of Defence]] and [[HM Forces]] public sites
* .net.uk: [[ISP]]s and [[Telecommunications company|network companies]] (unlike [[.net]], use is restricted to these users)
* .nhs.uk: [[National Health Service]] institutions
* .nic.uk: [http://www.nic.uk/registrars/becomeregistrar/taglist/ network use only] ([[Nominet UK]])
* .org.uk: general use (usually for [[non-profit organisation]]s)
* .parliament.uk: parliamentary use (only for the [[UK Parliament]] and the [[Scottish Parliament]])
* .plc.uk: [[public limited company|public limited companies]]
* .police.uk: [[UK police|police forces]]
* .sch.uk: [[Local education authority|Local Education Authorities]], [[school]]s, [[Primary education|primary]] and [[secondary education]], [[community education]]

=== Vanity domain ===
A [[vanity domain]] is a subdomain of an ISP's domain that is aliased to an individual user account, or a subdomain that expresses the individuality of the person on whose behalf it is registered.
<ref>{{cite news |last1=John |first1=Alex |title=Subdomain |url=https://thedomainrobot.com/blog/what-is-a-subdomain/ |access-date=29 November 2021 |archive-date=20 August 2024 |archive-url=https://web.archive.org/web/20240820200422/https://thedomainrobot.com/blog/what-is-a-subdomain/ |url-status=live }}</ref>
===Server cluster===
{{Redirect-distinguish|WWW2|World War II{{!}}WW2}}

Depending on application, a [[Resource record|record]] inside a domain, or subdomain might refer to a [[hostname]], or a service provided by a number of machines in a cluster. Some websites use different subdomains to point to different [[server cluster]]s. For example, <code>[[example.com|www.example.com]]</code> points to Server Cluster 1 or Datacentre 1, and <code>www2.example.com</code> points to Server Cluster 2 or Datacentre 2 etc.

====Subdomains versus directories====
Subdomains are different from directories. Directories are physical folders on an actual computer, while subdomains are a part of the [[URL]] that can be routed to any file or folder on the server machine.

==See also==
*[[Domain name]]
*[[Hostname]]
*[[Subdirectory]]
*[[Subpage]]
*[[Vanity domain]]
*[[Webpage]]

==References==
{{reflist}}

[[Category:Domain Name System]]