IP addresses are difficult for humans to remember, they're great for PCs! Domain names were invented to make it easier to navigate the Internet. A domain name is a vaguely descriptive name separated by dots. For example: www.linuxhq.org
Every machine that runs TCP/IP has a text file called hosts. It is a simple lookup table that the network stack (IP) checks to see if it has a match between a domain name and an IP address. It is easily modified with a text editor and the contents look like the following:
127.0.0.1 localhost 18.104.22.168 e237-firewall.tech.el.sait.ab.ca 22.214.171.124 e237-bridge.tech.el.sait.ab.ca 126.96.36.199 ashley.tech.el.sait.ab.ca 188.8.131.52 mariah mariah.tech.el.sait.ab.ca
The IP address is listed on the left and the domain name is listed on the right. The actual registered domain name is sait.ab.ca (Southern Alberta Institute of Technology). The domain name el.sait.ab.ca (electronics dept.) is a subnet of sait.ab.ca. The domain name tech.el.sait.ab.ca (technical) is a subnet of el.sait.ab.ca.
The machine names are e237-firewall, e237-bridge, ashley and mariah. Mariah's entry is unique in that both the domain name mariah and mariah.tech.el.sait.ab.ca would be recognized by the IP stack as 184.108.40.206.
The problem with the hosts file is that each machine must have a current up to date copy of the network. For a small network (25 or less) not connected to the Internet this is not a problem to manage. If the network is larger, than problems can occur trying to keep everyone updated.
Another solution is Unix's Network Information Service (NIS) (formerly called Yellow Pages until there was a copyright conflict with the Telcos). A central NIS server shares a master hosts file to all the clients. In this way, only one file exists and is updated. This works well for a network not connected to the Internet.
If you are connected to the Internet then a Domain Name Server (DNS) is used. A DNS is a special server that communicates with other servers and keeps an up-to-date look-up table that matches IP addresses to domain names for the complete Internet. It is a hierarchical system where each DNS is authorative for the domain underneath it. This means that each server knows the domain name to IP address mapping of the network underneath it.
Domain Name Structure
Domain names tend to follow a loose structure that gives a description of the network. For example, sait.ab.ca uses the Canada extension .ca, preceded by the province of Alberta extension ab and then the abbreviation SAIT for the Southern Alberta Institute of Technology. This is a geographical designed domain name that follows the ISO-3166 country code structure as listed in ISO 3166 Country Codes. Example of country codes are:
br Brazil ca Canada fi Finland gb United Kingdom na Nambia nz New Zealand tw Taiwan us United States
There are top level domain (TLD) names extensions that attempt to describe the purpose of the domain. It is broken down into seven basic categories:
com - Commercial domains that are a business. edu - Educational institutes net - This is for computers of network providers such as Internet Service Providers org - Standard organizations or non profit organizations int - Organizations that have been established by international treaties. gov - Municipal, federal, provincial, state governments. mil - United States military
All in all, it is often quite difficult to establish where a domain is physically located or what it actually does from the domain name. But it makes remembering locations easier than attempting to remembering an IP address.
Domain Name Look-up Procedure
When a domain name is used, the IP stack doesn't understand domain names. It says "what is this? Is not an IP address!". The only thing the IP stack understands is IP addresses. The look-up order is as follows:
On the Internet, there are 13 top level root domain name servers. The current addresses and domain names are found at ftp://internic.net/domain/named.cache (also called named.ca and named.root)
Domain Name IP Address Description A.ROOT-SERVERS.NET 220.127.116.11 formerly NS.INTERNIC.NET B.ROOT-SERVERS.NET 18.104.22.168 formerly NS1.ISI.EDU C.ROOT-SERVERS.NET 22.214.171.124 formerly C.PSI.NET D.ROOT-SERVERS.NET 126.96.36.199 formerly TERP.UMD.EDU E.ROOT-SERVERS.NET 188.8.131.52 formerly NS.NASA.GOV F.ROOT-SERVERS.NET 184.108.40.206 formerly NS.ISC.ORG G.ROOT-SERVERS.NET 220.127.116.11 formerly NS.NIC.DDN.MIL H.ROOT-SERVERS.NET 18.104.22.168 formerly AOS.ARL.ARMY.MIL I.ROOT-SERVERS.NET 22.214.171.124 formerly NIC.NORDU.NET J.ROOT-SERVERS.NET 126.96.36.199 temporarily housed at NSI (InterNIC) K.ROOT-SERVERS.NET 188.8.131.52 housed in LINX, operated by RIPE NCC L.ROOT-SERVERS.NET 184.108.40.206 temporarily housed at ISI (IANA) M.ROOT-SERVERS.NET 220.127.116.11 housed in Japan, operated by WIDE
These are controlled by InterNIC which is the primary agency responsible for registering domain names. At the time of this writing, there are several new agencies that are taking over the domain registration process for different parts of the world.
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