A third-level domain is the next highest level in the domain name hierarchy below the second level domain and the segment that appears directly to the left of the second level domain.
Domain names are names that uniquely identifies a site (e.g., a web site or ftp site) on the Internet or other TCP/IP (transmission control protocol/Internet protocol) networks and serve as user-friendly substitutes for difficult-to-remember, numeric IP addresses.
A second-level domain is the portion of a domain name directly below the top level domain (TLD) in the domain name system (DNS) hierarchy and the portion that appears immediately to the left of the TLD. The TLD is the last part of the domain name, that is, the segment furthest to the right, such as .com, .org and .edu, or the two segments furthest to the right if a two-letter country code (ccTLDs) is also used.
For example, the www in www.was.org is a third-level domain. .org is a top level domain and was is a second-level domain. The www would still be the third-level domain even if it were the fourth segment of the domain name because of the addition of a ccTLD, for example www.was.com.my.
The third level domain is typically used to refer to a specific server within an organization. In larger organizations each division or department might have its own third level domain, which can serve as an effective means for identifying it. www is the default third-level domain name and the most common by far.
Extending the above example, if was.com had an FTP (file transfer protocol) server for allowing users to download files, its third-level domain name could be called ftp and its full domain name would be ftp.was.com. Likewise, the domain names support.was.org and members.was.org could be used to distinguish support services and member services provided by was.com and to direct web traffic accordingly.
Multiple third-level domain names are also used for the purpose of load balancing (i.e., allocating traffic among identical servers at the same location) for sites that receive a large amount of traffic. Sometimes names such as www1 and www2 are used for this purpose.
Third-level domain names are not required. Thus, it would be possible to have a fully functional domain name such as was.org. All that is required is two levels: the TLD and a second-level domain name. However, the use of third-level domain names can usually add clarity to domain names and make them more user-friendly.
Third level domains should not be confused with the part of e-mail addresses that appears to the left of the at symbol. Thus, for example, the support in firstname.lastname@example.org is not a third level domain, but rather is merely a specific addressee or mailbox at an Internet address.