In the Domain Name System (DNS) hierarchy, a second-level domain (SLD or 2LD) is a domain that is directly below a top-level domain (TLD). For example, in example.com, example is the second-level domain of the .com TLD. Second-level domains commonly refer to the organization that registered the domain name with a domain name registrar. Some domain name registries introduce a second-level hierarchy to a TLD that indicates the type of entity intended to register an SLD under it. For example, in the .uk namespace a college or other academic institution would register under the .ac.uk ccSLD, while companies would register under .co.uk. Strictly speaking, domains like and are second level domain themselves, since they are right below a TLD. A list of the official TLDs can be found at icann.org and iana.org. An ordinal-free term to denote domains under which people can register their own domain name is public suffix domain (PSD).

Country-code second-level domains

Algeria

Australia

Austria

In Austria there are two second-level domains available for the public: The second-level domain

Bangladesh

Brazil

France

In France, there are various second-level domains available for certain sectors, including

Hungary

New Zealand

Nigeria

Pakistan

India

, liberalised policies for the .in domain allow unlimited second-level registrations under .in. Unlimited registrations under the previously structured existing zones are also allowed: Zones reserved for use by qualified institutions in India: Before the introduction of liberalised registration policies for the .in domain, only 7000 names had been registered between 1992 and 2004. , the number had increased to over 610,000 domain names with 60% of registrations coming from India and the rest from overseas. By October 2011, the number had surpassed 1 million domain names. , the number has more than doubled to over 2 million domain names.

Israel

Japan

Russia

South Africa

South Korea

Spain

Sri Lanka

Registrations are taken at the second level and also at the third level beneath various categorized second level names. A second-level registration automatically blocks the name from registration by anybody else under any of the third-level names.

Thailand

Trinidad and Tobago

Türkiye

In Türkiye, domain registrations, including the registration of second-level domains is administrated by TRABİS. There 22 active second-level domains under the .tr TLD. 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. Türkiye second-level domains include:

Ukraine

Ukraine second-level domains include: There are also numerous geographic names.

United Kingdom

United States

A two-letter second-level domain is formally reserved for each U.S. state, federal territory, and the District of Columbia.

Zambia

Historic second-level domains

There are several second-level domains which are no longer available.

Australia

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.

Canada

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., Calgary Board of Education www.cbe.ab.ca) and others were moved to more traditional subdomains (www.transportation.alberta.ca).

France

Historic second-level domains for France included: .tm.fr (for brands), .com.fr (for commercial use) and .asso.fr.

The Netherlands

Historic second-level domains for the Netherlands included: .co.nl (for commercial use)

Yugoslavia

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.

Tuvalu

Historic second-level domains for Tuvalu included: co.tv

Legal issues

As a result of ICANN's generic top-level domain (gTLD) expansion, the risk of domain squatting has increased significantly. For example, based on current regulations, the registration of the gTLDs .olympics or .redcross is not allowed; however, the registration of sites such as olympics.example or redcross.example is not controlled. Experts say that further restrictions are needed for second-level domains under the new gTLD .health, as well. For example, second-level domains under .tobacco.health or .diet.health can be easily misused by companies and therefore are a potential threat to Internet users.