DNS (Domain Name System but aka Domain Name Service; Domain Name Server;). A system that resolves/translates user-friendly names (such as host names or domain names) into computer-friendly names (such as IP addresses). A DNS can be a single server on a LAN or the multitude of DNS servers distributed all over the Internet. If one DNS server can't resolve a domain name it can ask other DNS servers as needed.
This page is concerned with Internet domains and DNS as opposed to LAN equivalents.
There are many different ways of identifying a computer, but for our purposes there are really two:
- MAC address (Media Access Control address). A MAC address is a 12 digit hexadecimal code stored in the ROM chip of a NIC (Network Interface Card). A computer is uniquely identified on a network by its MAC address in the MAC sub-layer of the OSI Data Link Layer and the OSI Physical Layer. There are 16^12 or approximately 2.8e14 possible values for a 12 digit hexadecimal number.
- IP address (Internet Protocol address). An IP address is a 32 digit binary code that uniquely identifies a computer on a network in the OSI Network Layer as part of the TCP/IP protocol suite. The IP address is also commonly represented as 4 octets of binary numbers or a 4 decimal numbers, eg 184.108.40.206. There are 2^32 or 256^4 or approximately 4.3 billion possible values for a 32 digit binary number. IP addresses are routable. See also my article on TCP/IP.
All other names and such eventually translate into a MAC or an IP address or both.
Reverse DNS resolves/translates IP addresses into domain names.
A DNS name (aka the usual domain name) is a user-friendly name equivalent to an IP address of a unique server on the Internet. The DNS naming convention is hierarchical and structured.
A DNS name must be a FQDN (fully-qualified domain name) in order to translate into an IP address. A DNS address consists of "labels" (a set of letters, numbers, and hyphens up to 63 characters), each separated by a "dot" (a period). Lately non-ASCII characters have been allowed (as encoded Unicode) but it's still buggy.
The labels of a FQDN are as follows:
- The rightmost portion of a domain is called a TLD (top-level domain).
- One or more subdomains follow the TLD.
- The leftmost portion of a domain is the hostname of the server. This is often the same as the NetBIOS Name in Windows networks
The rightmost portion of a domain name is called a TLD (top level domain).
gTLDs (Generic TLDs) have three or more characters and are supposed to be by kind of organization. There are currently16 gTLDs maintained by ICANN (Internet Corporation for Assigned Names and Numbers) [2005-04]. The first six gTLDs are in bold.
ccTLDs (Country Code TLDs) have two characters and usually match ISO 3166. Personally I would have preferred the three letter codes. EGs: These were chosen out of the list for personal quirky reasons.
*Some countries allow foreign entities to use their ccTLD.