Amer Networks SS3GR24i - Manual (Page 46)

SS3GR24i/SS3GR48i Layer3 Stackable Gigabit Switch User’s Guide
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The same computers are making the connections, so the IP
addresses are the same. Both computers are using the same well-
known TCP port for the FTP server. The local FTP clients are
using different TCP port numbers.
Connection 2 10.42.73.23, 10.128.12.1 1235, 21
Connection 1 10.42.73.23, 10.128.12.1 1234, 21
Internet addresses TCP ports
No two TCP connection can have the same set of numbers, but only
one number needs to be different. It is possible, for example, for
two users to send files to the same destination at the same time.
This could give the following connection numbers:
A TCP connection is then described by a set of four numbers – the
IP address and TCP port number for the local computer, and the IP
address and TCP port number for the remote computer. The IP
address is in the IP header and the TCP port number is in the TCP
header.
TCP will open a connection to the FTP server using some random
port number, 1234 for example, on the local computer. TCP will
specify port 21 for the FTP server. Port 21 is the well-known port
number for FTP servers. Note that there are two different FTP
programs running in this example – an FTP client that requests
the file to be transferred, and an FTP server that sends the file to
the FTP client. The FTP server accepts commands from the client,
so the FTP client must know how to connect to the server (must
know the TCP port number) in order to send commands. The FTP
Server can use any TCP port number to send the file, so long as it
is sent as part of the connection setup.
This is accomplished using sockets. A socket is a pair of TCP port
numbers used to establish a connection from one computer to
another. TCP uses these port numbers to keep track of connections.
Specific port numbers are assigned to applications that wait for
requests. These port numbers are referred to as ‘well-known’ ports.