TCP/IP Communication

The TCP/IP process for two computers to communicate over a network breaks down into four distinct steps. The four steps the TCP/IP protocol takes on a sending host before sending out a packet are:

1. Resolves the host name or NetBIOS name to an IP address.
2. Using the destination IP address and the IP routing table, TCP/IP determines the interface to use and the forwarding IP address.
3. For unicast IP traffic on shared access technologies such as Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI), ARP resolves the forwarding IP address to a MAC address.

   For multicast IP traffic on Ethernet and FDDI, the destination multicast IP address is mapped to the appropriate multicast MAC address. For multicast IP traffic on Token Ring, the functional address of 0xC0-00-00-04-00-00 is used. For broadcast traffic on shared access technologies, the MAC address is mapped to OxFF-FF-FF-FF-FF-FF.

4. The IP datagram is sent to the MAC address resolved through ARP or through the multicast mapping.

The following sections describe each portion of this process. The TCP/IP stack always follows this sequence when determining how to get a packet from point to point. To skip directly to the standard troubleshooting sequence, see the "Unable to Resolve a Host or NetBIOS Name" section of this chapter.

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