Internet  Mail Access Protocol

Another  mail access  protocols  is internet  mail access  protocols  version 4 (  IMAP4). IMAP4 is  similar to POP3 but it  has more  feature  IMAP4  is more  powerful  and more  complex.

POP3 is deficient  is several ways. It does not allow the user to organize her mail on the  server the user cannot  have different  folders on the sever. In addition POP3 does not allow the user to partially check the contents of the mail  before  downloading.

IMAP4  provides  the followings  extra functions:

a.A user  can check  the e mail  header  prior  to downloading .

b.A user can  search the contents  of the e mail  for a specific string  of characters prior  to downloading.

c.A user can  partially download email. This is especially  useful  if bandwidth  is limited  and the  email contains  multimedia  with high  bandwidth  requirements.

d.A user can  create  delete  or rename  mailbox on the  mail server.

e. A user can create a hierarchy of  mailboxes   in a folder for email storage.

A comparison of POP3 IMAP is given in table

Domain Names Server (DNS) The internet Directory  Service

The are several  application in the application  layer of the internet  model that  follow  the client  server paradigm. The client  server programs can be divided  into  two  categories those that  can be  directly  used by  the user such as e may and those that  support other application programs. The domain  name systems is a supporting programs that is used by  programs such as  e mail.

The domain name systems is a hierarchical naming systems for computers services or any  participating the internet. it associates various  information meaningful  domain  names to the numerical ( Binary) identifiers associated with  networking  equipment for eh purpose of locating and addressing these devices world  wide. An often  used  analogy to explain  the domain  name  systems is that is serves as the phone  book  for the  internet  by translating human friendly  computer  hostnames into IP address for example  www. Example. Com  translates to 208.77.188.166.

Figure shows  an example  of how  a DNS client / Server program  can support  an email program to  find the IP address of an email recipient. A user of an email  program may know the email address of the recipient however the IP protocol needs the IP address.

The DNS client  program  sends a request to a DNS server to map the  email address to the corresponding  IP address.

To identify an entity TCP IP protocol  use the  IP address which  uniquely identifies the connection of a host to the internet.

An IP address consists of four bytes and has a rigid hierarchical  structure. An  address looks like 121.7.106.83where  each period separates one of  the bytes expressed in decimal notion from 0 to 255.

However people  prefer to use names instead of numerical  addresses. Therefore  we need   a system  that can map  a name to  address or an  address to a name.

We have just  seen that there  are two  ways to identify  a host  by a host  a men add by an IP address. People prefer the more  mnemonic host name  identifier while router prefer fixed  length  hierarchically  structured  IP addresses. In order to reconcile these  is the  main task  of the internet domain name systems. The DNS is

a.A distributed  database  implemented in a hierarchy of DNS server and

b.An application layer  protocols  that allows  hosts  to query  the distributed  database.

DNS is commonly  employed by other application  layer protocols  including HTTP, SMIT and FTP to translate  user supplied  host names to IP  addresses. As an example  consider what  happens when a browser running  on some user host requests the URL www. Yahoo.com index.html. in  order for the  user host to be able to send and HTTP request message  to the  web server www. Yahoo.com  the user host must  first  obtain the IP address  of www. Yahoo .com this  is done of follow.

a.The same  user machine  runs the  client side of the DNS application.

b.The browser  extracts  the hostname www. Yahoo.com from the URL and passes the  hostname  to the client side of  the DNS applications.

c.The DNS  client  sends a query containing  the hostname  to a DNS server.

d.The DNS client  eventually  receives a reply  which  includes the IP address for the  host name.

e.Once  the browser receives the IP address  from DNS it can initiate  a TCP connection to the HTTP server process  located at the IP address.