D-Link DES-3326 User Manual - Udp and Icmp
DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
90
Switch Management and Operating Concepts
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:
Internet
addresses
TCP
ports
Connection 1 10.42.73.23, 10.128.12.1
1234, 21
Connection 2 10.42.73.23, 10.128.12.1
1235, 21
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. Once the commands are sent, a second 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’).
UDP and ICMP
There are many applications that do not require long messages
that cannot fit into a single packet. Looking up computer names is
an example. Users wanting to make connections to other
computers will usually use a name rather than the computer’s IP
or MAC address. The user’s computer must be able to determine
the remote computer’s address before a connection can be made. A