A grouping of TCP/IP hosts in the hierarchical Domain Name System (DNS). Domains are the building blocks of DNS. A domain consists of a group of nodes in the DNS namespace. Domains are organized hierarchically in the DNS namespace, with the topmost domain called the root domain.
DNS domains can be classified as one of the following:
Domain names can include only the characters a–z, A–Z, and 0–9, the dash (-), and the period. A name that completely identifies a host in the DNS namespace is called a fully qualified domain name (FQDN).