What are 3rd Level Domain Names?

Domain names are familiar, easy to remember addresses that allow people to easily locate web pages, email addresses, and network equipment host names. Domain names are defined in a hierarchy called the Domain Name System (DNS). In this system, the most specific host information is listed at the start of a domain name, and the more general host information is located at the end.

For example, Penn’s School of Design maintains a 3rd level domain name:
design.upenn.edu. This domain name is considered 3rd level because it provides three pieces of information to a user.

design = school designation, 3rd level
upenn = educational institution designation, 2nd level
edu = base domain, 1st level

Domain names can include even more information, creating 4th and even 5th level domain names. ISC Networking and Telecommunications registers and distributes domain name space for upenn.edu 3rd level domain names. Higher level domain names are the responsibility of individual schools and other University organizations.