D-Link DES-3326SR, User Manual

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DES-3326SR Layer 3 Fast Ethernet Switch User’s Guide
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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 designated computer on the
network will contain a database of computer names and their
corresponding IP and MAC addresses. The user’s computer
will send a query to the name database computer, and the
database computer will send a response. Both the query and
the response are very short. There is no need to divide the
query or response between multiple packets, so the complexity
of TCP is not required. If there is no response to the query
after a period of time, the query can simply be resent.
The User Datagram Protocol (UDP) is designed for
communications that do not require division among multiple
packets and subsequent reassembly. UDP does not keep track
of what is sent.