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Unit 12: Application Layer

12.1 Domain Name System (DNS) Notes

Now we have two types IP address in the form of decimal numbers and text for the same host.
You know that list of all IP addresses are maintained centrally by ICANN in the form of distributed
database directory. There are several distributed servers, which maintain this list of IP addresses.
The reasons behind the distributed server are very logical and simple. It helps in disaster
management and in diverting the load of the traffics in the form of requests from clients to other
DNS servers located at different sites. DNS server maintains database in both the form that is
textual as well as decimal notations. For example, DNS server maintains the address of google
site as www.google.com and 216.23.9.53.99. In this manner, DNS is used to provide host-to-IP
address mapping of remote hosts to the local hosts and vice versa. It is now amply clear that the
DNS maintains a distributed database to map between hostnames and IP addresses. Whenever
a client requests a service from a site, then both the site runs DNS protocol to access the distributed
database which is nothing but Domain Name Systems. Therefore, the DNS provides the protocol,
which allows clients and servers to communicate with each other. DNS enables a system to use
a resolver, which resolves the host name to IP address understandable by server.

You may be now thinking of how DNS is able to provide the quick translation of text of the IP
addresses within fraction of seconds from a directory of billions of such addresses. This could be
made possible by using Domain concepts, which uses hierarchical arrangements of text addresses
translation.

You can see from the Figure 12.1 that at the top level is the root server, which has null label.
Below this is another level domain or domain as com, edu, int and so on which are grouped
together. Below this different sub domains or groups have been created. Table 12.1 corresponds
to some commonly appearing domain names with their respective sites. The DNS can
accommodate almost all kinds of organizations by allowing each group to choose between
geographical or organizational naming hierarchies.

Figure 12.1: DNS Hierarchy

Root
Server

edu com int
google
hotmail
yahoo

Table 12.1: Internet Domains

Domain Indicative Site
Com Commercial institute
Edu Educational institute
Org Nonprofit organization
Net Network service provider
Gov Government department
Contd...

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