Second Level Domain
SLD is the acronym for Second Level Domain and represents 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. In the same way, the SLD represents the portion which is located exactly in the right part of the domain name extension. The SLD includes the TLD and helps during the identification process of a the owning organization of a specific URL.[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 are being 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" such as www.domainname.ab.com. It is important to make the distinction between top level domain, second level domains and other subdivisions.
Valid Domain Names and SLDs
In order to register a domain name, the following features must be met. In this way, a domain name is valid if it contains up to 67 characters including the domain name extension (such as .com). Valid characters 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]
ICANN and IDNA
By means of the IDNA system (Internationalized Domain Names in Applications) there is Unicode used into the valid DNS set of characters which permits the representation of names and words of languages in their native alphabets. In this way, the domain names can contain letters from non-ASCII scripts. The IDNA system was approved by ICANN, and was adopted by many registries that provide TLDs. [5]