Below the top-level domains in the domain name hierarchy are the second-level domain (SLD) names. These are the names directly to the left of .com, .net, and the other top-level domains. As an example, in the domain example.co.uk, co is the second-level domain.
Next are third-level domains, which are written immediately to the left of a second-level domain. There can be fourth- and fifth-level domains, and so on, with virtually no limitation. An example of an operational domain name with four levels of domain labels is sos.state.oh.us. Each label is separated by a full stop (dot). ‘sos’ is said to be a sub-domain of ‘state.oh.us’, and ‘state’ a sub-domain of ‘oh.us’, etc. In general, subdomains are domains subordinate to their parent domain. An example of very deep levels of subdomain ordering are the IPv6 reverse resolution DNS zones, e.g., 18.104.22.168.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa, which is the reverse DNS resolution domain name for the IP address of a loopback interface, or the localhost name.
Second-level (or lower-level, depending on the established parent hierarchy) domain names are often created based on the name of a company (e.g., bbc.co.uk), product or service (e.g. hotmail.com). Below these levels, the next domain name component has been used to designate a particular host server. Therefore, ftp.example.com might be an FTP server, www.example.com would be a World Wide Web server, and mail.example.com could be an email server, each intended to perform only the implied function. Modern technology allows multiple physical servers with either different (cf. load balancing) or even identical addresses (cf. anycast) to serve a single hostname or domain name, or multiple domain names to be served by a single computer. The latter is very popular in Web hosting service centers, where service providers host the websites of many organizations on just a few servers.
Country-code second-level domains
In Australia, currently there are 16 active second-level domains, all managed by auDA.
Open second-level domains (available for the public):
- .asn.au – for associations, political parties and clubs.
- .com.au – for commercial use.
- .net.au – for registered companies.
- .id.au – for Australian citizens only.
- .org.au – For non-profit organisations.
Closed second-level domains (restricted to certain sectors):
- .edu.au – for academic institutions.
- .gov.au – for government bodies.
- .csiro.au – for the Commonwealth Science and Industry Research Organisation (CSIRO). The domain is administrated by CSIRO itself.
Community Geographic Domain Names (restricted to certain geographic regions for community use):
- .act.au for the Australian Capital Territory.
- .nsw.au for New South Wales.
- .nt.au for the Northern Territory.
- .qld.au for Queensland.
- .sa.au for South Australia.
- .tas.au for Tasmania.
- .vic.au for Victoria.
- .wa.au for Western Australia.
Historically, Australia’s country code top-level domain was .oz. After the introduction of the .au ccTLD, the domains in .oz were moved under the oz.au second-level domain.
In Austria there are two second-level domains available for the public:
- .co.at intended for commercial enterprises
- .or.at intended for organizations.
The second-level domain
- .priv.at is restricted to Austrian citizens only, while
- .ac.at and .gv.at are reserved for educational institutions and governmental bodies respectively.
There are at least 66 second level domains:
In France, there are various second-level domains available for certain sectors, including
- .avocat.fr for attorneys,
- .aeroport.fr for airports and
- .veterinaire.fr for vets.
There are 21 active second-level domains in Hungary, including:
- .sport.hu and
The registration of second-level domains is managed by the Council of Hungarian internet providers.
Open second-level domains (available for the public):
- .nz – first level NZ domain, general use.
- .ac.nz – for tertiary educational institutions and related organisations.
- .co.nz – for commercial use.
- .geek.nz – For people who are concentrative, technically skilled and imaginative who are generally adept with computers.
- .gen.nz – General. For Individuals and other organisations not covered elsewhere
- .kiwi.nz – For people or organisations that associate with being ‘Kiwi’ (the colloquial term for New Zealanders)
- .maori.nz – Māori people, groups, and organisations.
- .net.nz – Organisations and service providers directly related to the NZ Internet
- .org.nz – For non-profit organisations.
- .school.nz – Primary, secondary and pre-schools and related organisations
Closed second-level domains (restricted to certain sectors):
- .cri.nz – for academic institutions.
- .govt.nz – for government bodies.
- .health.nz – for the Commonwealth Science and Industry Research Organisation (CSIRO). The domain is administrated by CSIRO itself.
- .iwi.nz – for traditional Māori tribes, hapū, or Taura here groups.
- .mil.nz – for the military organisation of the NZ Government.
- .parliament.nz – for parliamentary agencies, offices, political parties, and their elected members.
There are eight Second Level Domains.
- ac.il: Academic Institutions; administration delegated to the Inter-University Computation Center.
- co.il: Commercial Entities; administration delegated to five private registrars.
- org.il: Non-commercial Organizations; administration delegated to five private registrars.
- net.il: Israeli Internet service providers; available only to licensed providers by the Israel Internet Association.
- k12.il: Schools and Kindergartens
- gov.il: Government and Governmental System; administration delegated to ministry of finance.
- muni.il: Municipal Government
- idf.il: Israel Defense Forces; administration delegated to the army’s Center of Computing and Information Systems.
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Registration of other second-level domain names directly under .il is not supported.
Hebrew third level domains such as האינטרנט.org.il are available since 2010.
Second-level domain registrations are handled jointly by the official registry service CCTLDRU and private companies. There are currently 133 active second-level domains available for registration. This large number is because every geographical region has its own second-level domain, such .volgograd.ru for the Volgograd Region, .irkutsk.ru for the Irkutsk region or msk.ru for Moscow. There also second–level domains for specific sectors, such as ac.ru for academic institutions, com.ru for commercial enterprises or int.ru for international organizations.
Under the .za ccTLD there are several second-level domains in use. These include:
- .ac.za for academic institutions.
- .gov.za for government departments.
- .law.za for law firms and attorneys.
- .mil.za for the Department of Defence
- .nom.za for personal use. The registration is handled by NOM.ZA. and is available for free.
- .school.za for primary and secondary schools.
- .net.za for network providers.
Spanish second-level domains include .nom.es intended for personal names, .org.es for non-profit organizations and .gob.es for government agencies,com.es for companies.
In Turkey, domain registrations, including the registration of second-level domains is administrated by nic.tr. There 17 active second-level domains under the .trTLD. The registration of domains is restricted to Turkish individuals and businesses, or foreign companies with a business activity in Turkey. Second-level domains include .com.tr for commercial ventures, .edu.tr for academic institutions and .name.tr for personal use.
Ukraine second-level domains include:
- .gov.ua – available for government agencies.
- .com.ua – for commercial use.
- .in.ua – for commercial use.
- .org.ua – intended for non-profit organizations.
- .net.ua – available only for Internet providers in the UA.
- .edu.ua – for academic institutions.
… and geographic names: cherkassy.ua, cherkasy.ua, chernigov.ua, chernihiv.ua, chernivtsi.ua, chernovtsy.ua, ck.ua, cn.ua, cr.ua, crimea.ua, cv.ua, dn.ua, dnepropetrovsk.ua, dnipropetrovsk.ua, dominic.ua, donetsk.ua, dp.ua, if.ua, ivano-frankivsk.ua, kh.ua, kharkiv.ua, kharkov.ua, kherson.ua, khmelnitskiy.ua, khmelnytskyi.ua, kiev.ua, kirovograd.ua, km.ua, kr.ua, krym.ua, ks.ua, kv.ua, kyiv.ua, lg.ua, lt.ua, lugansk.ua, lutsk.ua, lv.ua, lviv.ua, mk.ua, mykolaiv.ua, nikolaev.ua, od.ua, odesa.ua, odessa.ua, pl.ua, poltava.ua, rivne.ua, rovno.ua, rv.ua, sb.ua, sebastopol.ua, sevastopol.ua, sm.ua, sumy.ua, te.ua, ternopil.ua, uz.ua, uzhgorod.ua, vinnica.ua, vinnytsia.ua, vn.ua, volyn.ua, yalta.ua, zaporizhzhe.ua, zaporizhzhia.ua, zhitomir.ua, zhytomyr.ua, zp.ua, zt.ua
Currently there are 12 active second-level domains under the .uk top-level domain. The majority of the domains is administrated by the UK’s domain registry services provider Nominet UK, while the others are managed by the British government. Generally, the registration of uk second-level domains is open to the public, however depending on the second-level domain there might by restrictions – for example .me.uk is open to the public, but .ac.uk is only available to educational institutions.
- .co.uk – intended for UK-based businesses. It is the UK’s most widely used second-level domain.
- .org.uk – intended for non-profit organizations.
- .me.uk – available only for personal use.
- .ltd.uk – available only for private limited companies in the UK.
- .plc.uk – available only for public limited companies in the UK.
- .net.uk – available only for Internet providers in the UK.
- .sch.uk – available only for schools in the UK.
- .ac.uk – intended for British academic institutions.
- .gov.uk – available for government agencies.
- .mod.uk and .mil.uk – available for the UK Armed Forces and Ministry of Defence.
- .nhs.uk – registration restricted to the National Health Service only.
- .police.uk – reserved for the UK police forces.
A two-letter second-level domain is formally reserved for each U.S. state, federal territory, and the District of Columbia:
Historic second-level domains
There are several second-level domains which are no longer available.
Second-level domains under .au which are no longer available include: .conf.au originally intended for conferences; .gw.au for the Australian Academic and Research networks; info.au for general information, .otc.au and .telememo.au for the X.400 mail systems.
Prior to 12 Oct 2010 there were second level domain based on province: .ab.ca — Alberta, .bc.ca — British Columbia, .mb.ca — Manitoba, .nb.ca — New Brunswick, .nf.ca — Newfoundland, .nl.ca — Newfoundland and Labrador, .ns.ca — Nova Scotia, .nt.ca — Northwest Territories, .nu.ca — Nunavut, .on.ca — Ontario, .pe.ca — Prince Edward Island, .qc.ca — Quebec, .sk.ca — Saskatchewan, .yk.ca — Yukon
Since 2010, some have been replaced (for example, alberta.ca) while others have remained under the provincial two letter SLD (e.g., transport Ontario www.mto.gov.on.ca) while others have been moved to more traditional subdomains www.transportation.alberta.ca).
Historic second-level domains for France included: .tm.fr (for brands), .com.fr (for commercial use) and .asso.fr.
Historic second-level domains for The Netherlands included: .co.nl (for commercial use)
In 2006 the .yu ccTLD was replaced by rs (for Serbia) and .me (for Montenegro). Second-level domains under .yu included: .ac.yu – for academic institutions, .co.yu for commercial enterprises; .org.yu for organizations and .cg.yu for residents of Montenegro. Only legal entities were allowed to register names under .yu and its second-level domains.
Historic second-level domains for Tuvalu included: co.tv