The Domain Name System (DNS) operates with a hierarchical linear structure. The domain .uk is what is known as a Top Level Domain, or TLD. Other TLDs amongst many, include .com, .net and .org. When reading a full domain name address, the TLD can always be found at the end of the address, on the far right hand side. For example:
A subdomain is a generic term that refers to all of the parts of a domain that can be found to the left of the TLD; the parts of the address that are not the TLD but form part of the larger domain name.
A subdomain can possess many parts. The part of the domain address that comes directly before the TLD is a Second Level Domain or SLD. The org in .org.uk and the co in .co.uk are both SLDs.
The domain name system allows for many subdomains in addition to the TLD and SLD. In practice domain names are not endlessly subdivided. However many organisations use subdomains to identify a particular department within a larger web address. For example, the colleges at Oxford University use subdomains. Somerville College’s domain name is some.ox.ac.uk and the University’s main website domain name is ox.ac.uk.