Technically, the second domain level is the data directly before the top-level domain TLD. For example, in www.domainname.com, “domainname” represents the second level domain. Whereas the suffix "(dot)-com" represents the TLD.
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. This can become confusing, as the domain name is commonly equated with the second level domain, and the subdomain is commonly equated with the third level domain.
A Functional SLD is a reasonable equivalent to an SLD in a namespace where domain names as second level domains are not permitted for policy reasons.
An example of a Functional SLD would be foo.com.au. While .com is the actual SLD, .au policy does not permit the widespread registration of second level domains, thereby creating a proliferation of Functional SLDs (in this case .foo) in the .au namespace.
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.
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.