A top-level domain (TLD) is the last part of an Internet domain name, which is the letters which follow the final dot of any domain name, for example, .com, .us, etc. TLD is assigned by the Internet Assigned Numbers Authority (IANA) and it is currently classifies into three types:

• generic top-level domains (gTLD): Used by a particular class of organizations (for example, .com for commercial organizations and .edu for an educational organizations). It is three or more letters long. Most gTLDs are available for use worldwide, but for historical reasons mil (military) and gov (governmental) are restricted to use by the respective U.S. authorities. gTLDs are subclassified into sponsored top-level domains (sTLD), e.g. .aero, .coop and .museum, and unsponsored top-level domains (uTLD), e.g. .biz, .info, .name and .pro.
• country code top-level domains (ccTLD): Used by a country or a dependent territory. It is two letters long, for example .us for United States and .cn for China.
• infrastructure top-level domains: The top-level domain arpa is the only confirmed one. Root has been known to exist without reason.