Page 191 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes
Mil Military
Biz Business
Country code For example, in for India, us for USA, au for Australia, jp for Japan and so on.

As we know that the servers maintaining addresses are distributed and have locations throughout
the world. Then this question arises as to how text addresses are organized in hierarchical
arrangement. You may refer Figure 12.1 above and Table 12.1. The hierarchy is represented into
zones and each zones is a hierarchy of one or more nodes without any overlapping. Each zone
is represented by a server and undoubtedly with one backup server. Root server as shown in
Figure 12.1 is only one, which is just indicative; there may be several root servers at several
locations in the world. Each root is aware of the location of each DNS server of specific domains.
The process is now very simple to understand. When you need to connect with a particular site,
you first send your request to your local host. If your local host can provide the translation, your
request is completed. If not, your local host then sends your request one level above in the
hierarchy. If the server at one level above is able to handle the same, you get your intended
website at your desktop through your local server. If not, then the server at one level above
from your local server either sends your request again to another server or informs your local
server that your request is failed and gives the address of another server to process your request.
This process continues till a server is found who knows the address, otherwise, the request is
filtered up to the root server. Depending upon the domain address, root server forwards the
request to the one of the domain servers represented at the next level of hierarchy. This process
continues and the information of text address is returned to Root server and then back to your
local server.

12.2 Electronic Mail

Electronic mail is one of the most popular network services. The use of Electronic mail, or e-mail
has probably may be cited as the foremost reason for the popularity of Internet. The proliferation
of cyber café can be credited to e-mail or World Wide Web. E-mail provides an efficient and fast
means of communication with relatives, friends or colleagues throughout the world. You cannot
only communicate with one person at a time or thousands but also you can receive and send files
and other information in a short time. In e-mail communication, the intended receiver or receivers
of the message are not required to be present at their desktop at the time of receiving of the
message by their computer. It works like a postal mail. In postal mail postman puts the sender’s
message in your mailbox and when you come back from your work, you access your mailbox to
retrieve the message. Therefore, we may consider it in a way a substitute of postal mail. However,
it has many-many more superior features than postal mail. E-mail has two parts:
User Agent: It is the user interface to the mail system. The user agent system enables to provide
ways to view, edit, and reply to messages, etc. It also accesses messages stored in a system
mailbox. The user agent enables the user to use a text editor to create a file that the user agent
hands over to the message transfer agent.
Message Transfer Agent (MTA): It is a software package that transports messages created by a
user to destination mailboxes possibly on remote machines. The MTA has to perform more
complex jobs than other applications:
1. MTA handles temporary failures when a destination machine is temporarily unavailable;
it stores the message on the local machine for later delivery. Thus, the User Agent typically
just stores messages into a storage area.

2. MTA distinguishes between local and remote recipients.

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