Second Level Domain
The Second Level Domain, or SLD, is generally the portion of the URL that identifies the actual website's domain name.[1]
Overview
Technically, the second domain level is the data directly before the top-level domain TLD.[2] For example, in www.domainname.com, “domainname” represents the second level domain. Whereas the suffix "(dot)-com" represents the TLD.
Usually, the SLD is determined and defined upon registration of a domain name. More than one SLD can function on the same IP Address.[3]
ccSLDs
A ccSLD, or Country Code Second Level Domain, refers to when an additional prefix is added to a ccTLD. An example of this would be .co.uk, where .uk is the (cc)TLD and .uk is the (cc)SLD. In this circumstance, the domain name ("example" in "example.co.uk") would be considered the third level domain, as it is in the third position.[4] This can become confusing, as the domain name is commonly considered to be the second level domain, and the subdomain is commonly considered the third level domain.
Valid Domain Names and SLDs
In order to register a domain name, the following features must be met:
- It may not contain more than 67 characters, including the domain name extension (such as .com).
- It must use valid characters, which are those between 0-9, letters a-z and dashes, but the dash "-" cannot be used at the beginning or at the end of a domain name.[5]
IDN
By means of the IDN system, the valid set of characters has been expanded to include names and words written in native scripts; in this way, the domain names can contain letters from non-ASCII scripts. The IDN fast track system was approved by ICANN in 2009, and was adopted by many registries that provide domain space.[6]