Networking / Beginners

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Network Domain Name Service

You need to identify each computer uniquely when computers are connected to each other in a network. Using IP addresses is the most widely accepted identification system today. Each computer is provided with a unique IP address. You can search for a computer by using its IP address.

I will discuss IP addresses in detail in the next section of the tutorial. However, the drawback of using an IP address is that it is difficult to remember. Consider how difficult it would be if you were required to remember the IP addresses of 50 computers. The alternative is to use the host names, which are easier to remember. For example, remembering www.yahoo.com is much easier than remembering the 32- bit IP address. However, you need a mechanism to convert host names to IP addresses, and vice versa. This mechanism is required because host names are ultimately converted to the IP addresses. The service that implements this mechanism is known as Domain Name Service (DNS).

In the early days of the Internet, the responsibility for maintaining unique host names for the computers on the Internet was given to the Stanford Research Institute's Network Information Center (SRI-NIC). Initially, they created a single file called hosts.txt that contained the host name to IP address mappings. All webmasters around the Internet were supposed to regularly update the SRINIC about any changes in status of their host names or IP addresses.

This created a lot of problems:

• The file grew with the growth of the Internet. After some time, it became extremely difficult to maintain the file.
• With the large number of computers connecting to the Internet, it became more impractical to guarantee the uniqueness of a host name.

These shortcomings created a need for things like hierarchical naming structure and distributed management of host names. This led to the evolution of a lookup facility known as DNS. It is a distributed database used for mapping host names to their respective IP addresses. The concept of the distributed database means a single organization will no longer be responsible for host name to IP address mappings.

The next few sections discuss basic concepts of the DNS.

IP Addresses

An IP address is a unique 32-bit name assigned to each computer in a network. A computer with an assigned IP address is known as a host. This IP address is used to identify hosts on a network.

The IP address consists of four 8-bit numbers separated by periods; for example, 172.17.20.10. This technique is known as dot-notation. A part of the address is used to identify the network to which the computer belongs. The remainder of the address identifies the system itself. Addresses are categorized under three classes:

• Class A
• Class B
• Class C

Class A is used to address very large networks, whereas class B addresses mediumsized networks. The class C addresses are assigned to small networks, with fewer than approximately 250 hosts.