Introduction to DNS
In a Nutshell - CIW Course Section 3, Part A1, Chapter 5
DNS Overview
In concept, a Domain Name System (DNS) is very simple. It takes a user-friendly, descriptive name and translates it into an IP address.
It may be simple, but it is absolutely essential to the running of the Internet. DNS is a hierarchical system with the domain names structured in a tiered fashion.
Domain Name Levels
The above example is not a domain name that is in use, so there is no point in trying to access it.
Root level domains are controlled by the Internet Assigned Numbers Authority (IANA) and are generally intended for specific purposes or geographic use. It appears however, that the rules about geographic usage are being relaxed. Anyone, for example, can use the ".it" domain implying Information Technology but being Italy in reality.
DNS Server
There are five types of DNS server, determined, mostly I think, by the way they are configured. These are: root server, master server, slave server, caching-only server and forwarding server. In reality, a server may perform more than one of these functions.
I have a DNS server running on a machine on my local home-network. This serves as a master server for my own domain, a forwarding server for most external domain requests, and as a caching server for recently visited domains.
Reverse Lookups
Occasionally, it is useful to be able to lookup an IP address and find the domain name associated with it. This is exactly what a reverse lookup does. To achieve this the DNS server will have a reverse lookup zone created and will use pointer (PTR) records instead of the forward lookup (A) records.
NSLOOKUP
This is a Unix utility that can run interactively to query DNS for domain name resolution and to view examples of DNS record types.
The nslookup utility uses the /etc/resolv.conf file to locate a name server.
Summary
DNS will be covered in much greater detail in part B of this section of the course. This is, after all, only an introduction.