D-Link DES-3326 User Manual page 112

24-port NWay Ethernet Switch User's Guide
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.
example, for two users to send files to the same destination at
the same time. This could give the following connection numbers:
Connection 1 10.42.73.23, 10.128.12.1
Connection 2 10.42.73.23, 10.128.12.1
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.
FTP transfers actually involve two different connections. The
connection begins by the FTP sending commands to send a
particular file.
connection is opened for the actual data transfer. Although it is
possible to send data on the same connection, it is very
convenient for the FTP client to be able to continue to send
commands (such as ' stop sending this file' ).
96
Internet addresses
Once the commands are sent, a second
Switch Management and Operating Concepts
It is possible, for
TCP ports
1234, 21
1235, 21