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7 layer OSI Model

The Transport Layer

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The IETF, IEEE, ISO, NIC and ANSI are some of the organization involved in standards creation. Standards are important because it ensures that a set of rules are in places so the different players (manufacturers) play the game with the same rules (it usually avoids chaos). SMTP, POP3, HTTP, FTP, TFTP, NTP and DNS are examples of popular applications which we use on day to day basis everything time we log onto the internet. Now like I said these are all application so I am talking about Layer7, Layer6 and Layer5 of the OSI layer.

But all the server applications need to listen for request from the users that is when they make use of the layer4 of the OSI layer. I mentioned that ports are entities defined at the transport layer (layer 4). If the client wants to access a certain application on the server it should access it on the correct port.

Lets say a server is running HTTP, FTP and SMTP/POP3 (We are talking high capacity server here), so there are 4 server applications running on the same machine. HTTP listens on port 80, FTP listens on port 21, SMTP listens on port 23 and POP3 listens on port 110. If you want to access a webpage on this server your machine will send a request to this server on port 80 so that the HTTP application comes into picture, if you want to access mails then your machine would send a request to the server of port 23/110 so the SMTP and POP3 application caters to your request.

These popular application have been assigned fixed ports to ensure that clients are able to access the required services on these ports. No matter where the web-server is being installed (America, Australia or Antarctica for that matter) it would listen on port 80 for HTTP requests. The web-server can obviously be designed to listen on a different port but then it would be one heck of a job to tell the whole world that "hey if you want to access my webpage you need to send your request on port 343434".

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