The Second Level Domain or SLD is an essential part of the hierarchical Domain Name System. It is the second part of the full domain name after the Top Level Domain, on its left side. The Second Level Domain is often the same as the website name, the company or the individual that registered it. There aren’t any restrictions for the registration of SLDs and you may choose any Second Level Domain for your website address and register it (unless somebody else already uses that domain). For example, in www.yourbrand.com there are three domain name levels: the first, the Top Level Domain is the .com part, the Second Level Domain is .yourbrand and the Domain on the third level is the www segment.
Types Of Second-Level Domains
So, although the SLD is the most important part of your domain name that you can usually choose freely, there are some Second Level Domain names that have to be used as additional classes for some country-code TLDs. These are used by the United Kingdom (they are put in front of the .uk TLD) and Brazil (for the .br TLD). If you want to register a domain name in the U.K. you must add any of the predefined ccSLDs: .ac.uk, .co.uk, .gov.uk, .org.uk or others. Brazil also requires a ccSLD before the .br and for websites ending with TLDs like .com.br, .net.br, .eco.br or .edu.br your website name (the domain that you actually register under the TLD) will actually be the Third Level Domain.