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24-port NWay Ethernet Switch User’s Guide
Switch Management and Operating Concepts 95
header and checks the checksum. It then looks at the type code.
If the type code is for IP, the packet is given to IP. IP then
removes the IP header and looks at its protocol field. If the
protocol field is TCP, the packet is sent to TCP. TCP then looks
at the sequence number and uses this number and other data
from the headers to reassemble the data into the original file.
Well-Known Sockets and the Application
Layer
Application protocols run ‘on top of’ TCP/IP. When an
application wants to send data or a message, it gives the data to
TCP. Because TCP and IP take care of the networking details,
the application can look at the network connection as a simple
data stream.
To transfer a file across a network using the File Transfer
Protocol (FTP), a connection must first be established. The
computer requesting the file transfer must connect specifically to
the FTP server on the computer that has the file.
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.
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