Internet/E-commerce- Transmission Control Protocol/ Internet Protocol (TCP/IP) Tutorial

         1   2 3 ...   n                   1   2 3 ...   n
          \  |      /      |               \  | |      /       ^
           \ | |   /       |                \ | |     /        |
         -------------   flow              ----------------   flow
         |multiplexer|    of               |de-multiplexer|    of
         -------------   data              ----------------   data
              |            |                     |              |
              |            v                     |              |
              1                                  1

        Figure 2.  n-to-1 multiplexer and 1-to-n de-multiplexer

   If an Ethernet frame comes up into the Ethernet driver off the
   network, the packet can be passed upwards to either the ARP (Address
   Resolution Protocol) module or to the IP (Internet Protocol) module.
   The value of the type field in the Ethernet frame determines whether
   the Ethernet frame is passed to the ARP or the IP module.

   If an IP packet comes up into IP, the unit of data is passed upwards
   to either TCP or UDP, as determined by the value of the protocol
   field in the IP header.

   If the UDP datagram comes up into UDP, the application message is
   passed upwards to the network application based on the value of the
   port field in the UDP header.  If the TCP message comes up into TCP,
   the application message is passed upwards to the network application
   based on the value of the port field in the TCP header.

   The downwards multiplexing is simple to perform because from each
   starting point there is only the one downward path; each protocol
   module adds its header information so the packet can be de-
   multiplexed at the destination computer.

   Data passing out from the applications through either TCP or UDP
   converges on the IP module and is sent downwards through the lower
   network interface driver.

   Although internet technology supports many different network media,
   Ethernet is used for all examples in this tutorial because it is the
   most common physical network used under IP.  The computer in Figure 1
   has a single Ethernet connection.  The 6-byte Ethernet address is
   unique for each interface on an Ethernet and is located at the lower
   interface of the Ethernet driver.

   The computer also has a 4-byte IP address.  This address is located
   at the lower interface to the IP module.  The IP address must be
   unique for an internet

Source: http://tools.ietf.org/