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 188.8.131.52.
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 184.108.40.206where 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.