Figure shows the location of the source and destination port numbers within a segment.  The figure is showing a computer that has both an active FTP session and a web session.  When data is meant for the FTP server the computer selects a random source port value above 1023 and assigns 21 to the destination port.  In the figure the source port used is 1305.  The IP address and the port number create the socket and this allows the computer to keep track of the FTP data.  The web traffic also uses a random source port above 1023, and in the figure it is using port 1099.  The data destined for the web server is assigned a destination port of 80, which is the port used by web servers.  The IP address and the port number create the socket and this allows the computer to keep track of the web server connection.