Enterasys VH-2402-L3 Management Manual page 164

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.
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.
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.
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:
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.
156 VH-2402-L3 Management Guide
TCP ports
Connection 1
1234, 21
Connection 2
1235, 21
Internet addresses
10.42.73.23, 10.128.12.1
10.42.73.23, 10.128.12.1
9033691-01