Domain has multiple related meanings: A host name which identifies a computer or computers on the Internet. These names appear as a part of a Web site's URL, e.g. en.hostname.org; a domain name that registrars are provided with.
These are called registered domain names; Special names that follows the @ sign in an email address, Top-level domain names like .com, names used by Session Initiation Protocol (VoIP), Domain keys are examples of Names used for other purposes in the Domain Name System (DNS); Often times referred as "web addresses".
Common domain names are host names that are provided that we can easily remember rather than numeric IP addresses. Because of the unique alphabetical addresses, Internet users have an easier time finding and communicating with web sites and other server-based services.
The domain name allows multiple IP addresses to be assigned to a single domain name, or multiple domain names to be assigned to a single IP address, allowing servers to be flexible. Several servers can also assign themselves to a single IP address.
A hostname is restricted to ASCII letters; these are "a" through "z", numerical digits "0" through "9", and hyphen, with some restrictions. The Internationalized domain name (IDN) system bypasses restrictions on character allowances in host names. This makes it easier to use non-English alphabets on the Internet.
Domain names end in a top-level domain (TLD) name. The list can be of generic names (3 or more characters) or two-character territory codes. Top-level domains are also called first level domains. Examples are .com, .co.uk, edu, .gov, .info, .travel, .tel, .us, .uk, .au , .nz etc.
There are also second-level domains (SLD). These are the names left of .com, .net and other top-level domain names. Further, there are also third level domains which is immediately at the left of the second-level domain.
There can also be four to five-level domains, e.g. www.sos.state.oh.us. A dot will separate each level. Third-level domains and higher can also be called "sub domains". Second-level domains are based on the name of a company. However, sub domains may still not serve any real purpose at all other than simply seeing the name of the web site.
