Introduction to Data Communications
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54e. Domain Names

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

	142.110.237.1	e237-firewall.tech.el.sait.ab.ca

	142.110.237.2	e237-bridge.tech.el.sait.ab.ca

	142.110.237.3	ashley.tech.el.sait.ab.ca

	142.110.237.4	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 142.110.237.4.

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.

Introduction to Data Communications
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