The Second Level Domain, or SLD, is the portion of the URL that identifies the specific and unique owner. The SLD is the actual website's domain name.[1]
Overview
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]
Furthermore, a TLD can be separated into more SLDs; that is, second-level domains can be divided into sub-domains which could represent different computers within an organization or different aliases.
There are also third-level domain names referred to as "provincial" domains, such as www.domainname.ab.com.
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.[4]
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.[5]