What is Application Layer

What is application layer
Introduction:- application layer is the outermost layer of the tCP / IP architecture. This layer is responsible for many of the user applications such as www kumar email, fTP, DNS etc, in this chapter The Reader will get a basic understanding of the concept of some popular application layer functions. The the lower layers tCP / IP model does support for Transport. However these is still need for some transport functions at application layer which an essential for application to run.14 the most important one is DNS.

Domain name system (DNS):- it is well known that the IP addresses are used to identify the devices in the internet such as routers servers etc. In the absence of a domain name for an email server we would have ended which a representation such as aBC @ 144 dot 16.7 0.2, y z @ 20 2.16 707 2.16 etc It can been seen that such a representation is very difficult to remember that to impossible if there are hundreds of such email ids. Is loaded to a different machine with a different IP address the above scheme does not work. If this is the case with email then how about the thousands of websites for example http:// 20 2.1 6.7 0.1~ index.html is a URL we need to remember the entire number to access the page. Those it is clear that the IP addresses are difficult to remember and DNS is a perfect solution to this problem.
Some points on DNS
It is a hierarchical scheme
It employees domain based naming
It uses a distributed database
Primarily maps hostnames and email server to IP addresses
Found in RFC 1034, 1035
Working of DNS
An application program calls a library procedure called resolver with its domain name as parameter
The resolver stands and udp packed to the local DNS server. The DNS server search its table and returns the IP address which match the domain name armed with that, the program can establish haccp connection or send udp packet.