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A second-level domain is the label immediately to the left of a top-level domain (TLD). In example.com, example is the second-level label and .com is the TLD. The term can also be used for the full name example.com, so context matters.
What is a second-level domain?
Domain names follow a hierarchy, with labels separated by dots. The last named level is the top-level domain; the label immediately before it is the second-level label. ICANN’s glossary defines a second-level domain as “A label that is found in a zone that is under a top-level domain.” ICANN glossary
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How to identify each level
Read the name from right to left to identify its levels:
- In
icann.org,orgis the top-level domain (TLD). icannis the second-level label, directly beneathorg.- In
whois.icann.org,whoisis a third-level label, also commonly described as a subdomain.
ICANN uses whois.icann.org to illustrate this hierarchy: org is the top level, icann the second level, and whois the third. ICANN domain-name registration process
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Does “second-level domain” mean a label or a whole domain name?
Usage is not always consistent. Strictly, the second-level domain is the individual label before the TLD—for example, icann. ICANN also calls the full name icann.org a “second-level name.” Its FAQ describes the portion before the TLD as the second-level domain name, using icann in icann.org as its example. ICANN FAQs
When precision matters, say whether you mean the label (icann) or the full registered name (icann.org).
Is a subdomain the same as a second-level domain?
No. A subdomain label appears to the left of the second-level label. In whois.icann.org, whois is the subdomain label, while icann remains the second-level label and org the TLD. A subdomain therefore adds another level; it does not replace the second-level label.
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