In data networks, devices are labeled with numeric IP addresses to send and receive data over networks. Most people cannot remember this numeric address. Domain names were created to convert the numeric address into a simple, recognizable name.
On the Internet, these domain names, such as http://www.cisco.com, are much easier for people to remember than 198.133.219.25, which is the actual numeric address for this server. If Cisco decides to change the numeric address of www.cisco.com, it is transparent to the user, because the domain name remains the same. The new address is simply linked to the existing domain name and connectivity is maintained. When networks were small, it was a simple task to maintain the mapping between domain names and the addresses they represented. As networks have grown and the number of devices increased, this manual system became unworkable.
The Domain Name System (DNS) was created for domain name to address resolution for these networks. DNS uses a distributed set of servers to resolve the names associated with these numbered addresses. Click the buttons in the figure to see the steps to resolve DNS addresses.
The DNS protocol defines an automated service that matches resource names with the required numeric network address. It includes the format for queries, responses, and data. The DNS protocol communications use a single format called a message. This message format is used for all types of client queries and server responses, error messages, and the transfer of resource record information between servers.
Figures 1 through 5 display the steps involved in DNS resolution.