what is a domain?

Answer 1

in the context of networking, a "domain" is a collection of clients, servers, and networking devices all under a single source of administrative control.

thats really about the best definition because all of the more granular definitions may or may not be applicable in different context.

Answer 2

A name by which a computer connected to the Internet is identified. A typical domain name looks like this: www.ibm.com. The "www." refers to the fact that this computer is connected to the World Wide Web; the middle portion of a domain name is usually the name of the company that owns the computer—in this case,IMB ; the final portion of a domain name tells you what kind of site is served by this machine—in this case, ‘.com’ means this is a commercial site (other types of sites are: .edu—education, .org—non-profit organization, .net—Internet service provider).

Answer 3

A top-level domain (TLD) is the last part of an Internet domain name; that is, the letters which follow the final dot of any domain name. For example, in the domain name en.wikipedia.org, the top-level domain is org (or ORG, as domain names are not case-sensitive).

The Internet Assigned Numbers Authority (IANA) currently classifies top-level domains into three types:

* country code top-level domains (ccTLD): Used by a country or a dependent territory. It is two letters long, for example jp for Japan.
* generic top-level domain (gTLD): Used (at least in theory) by a particular class of organizations (for example, com for commercial 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.
* infrastructure top-level domain: The top-level domain arpa is the only confirmed one. Root has been known to exist without reason.
The term domain name has multiple meanings, all related to the:

Main article: Domain name system

* a name that is entered into a computer (e.g. as part of a Web site or other URL, or an e-mail address) and then looked up in the global Domain Name System which informs the computer of the IP address(es) with that name.
* the product that registrars provide to their customers.
* a name looked up in the DNS for other purposes.

They are sometimes colloquially (and incorrectly) referred to by marketers as "Web addresses".

* The authoritative definition is that given in the RFCs that define the DNS.

Domain names are hostnames that provide more easily memorable names to stand in for numeric IP addresses. They allow for any service to move to a different location in the topology of the Internet (or another internet), which would then have a different IP address.

Each string of letters, digits and hyphens between the dots is called a label in the parlance of the domain name system (DNS). Valid labels are subject to certain rules, which have relaxed over the course of time. Originally labels must start with a letter, and end with a letter or digit; any intervening characters may be letters, digits, or hyphens. Labels must be between 1 and 63 characters long (inclusive). Letters are ASCII A–Z and a–z; domain names are compared case-insensitively. Later it became permissible for labels to commence with a digit (but not for domain names to be entirely numeric), and for labels to contain internal underscores, but support for such domain names is uneven. These are the rules imposed by the way names are looked up ("resolved") by DNS. Some top level domains (see below) impose more rules, such as a longer minimum length, on some labels. Fully qualified domain names (FQDNs) are sometimes written with a final dot.

Translating numeric addresses to alphabetical ones, domain names allow Internet users to localize and visit Web sites. Additionally since more than one IP address can be assigned to a domain name, and more than one domain name assigned to an IP address, one server can have multiple roles, and one role can be spread among multiple servers. One IP address can even be assigned to several servers, such as with anycast and hijacked IP space.