WindowsNetworking.com - When one thinks of the Internet, the first thing that comes to mind is surfing from one Web site to another.
Being able to go from Web site to anothe, and view the contents is indeed the reason that the Internet is as popular as it is today. If we set Web surfing aside though, what do we have left in terms of actual usage going on while on the Internet?
One of the activities that takes place is the downloading of data files, movies, antivirus updates, and the like. What these acts have in common is one protocol, namely the FTP, or file transfer protocol.
It should be noted that FTP also observes the client/server model. Unlike HTTP, where there is a clear winner for Web browsers and Web servers, no such program can make the same claim as it relates to FTP. There is a large selection of FTP clients and servers out there today. It is worth noting that your version of Windows come with a built-in FTP client.
FTP itself uses the TCP transport protocol exclusively, or in other words, it never uses UDP for its transport needs. Typically an application layer protocol will use one or the other. One notable exception to that is DNS or Domain Name System. FTP also is odd in that it uses two ports to accomplish its task. It typically uses port 20 for data transfer and port 21 to listen to commands. However, having data transferred over port 20 is not always the case, as it can also be a different port as well.
That is where the confusing part for many people comes into play. There are two modes to FTP, namely active and passive mode. These two modes are initiated by the FTP client, and then acted upon by the FTP server.
Delving deeper
So how do active and passive FTP work? It all starts with the FTP client initiating a connection with the FTP server on its Port 21. Port 21 is where the server is listening for commands issued to it, and in turn, which it will respond to. So we will assume that the TCP/IP handshake is complete, and as normal, the client has done all of this on an ephemeral port.
At this point the client begins to listen on it's ephemeral port plus 1, and sends the PORT N+1 command to the server on its Port 21 i.e. if the ephemeral port in use by the client is 1026, then it would
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