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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Network Interface / Internet "Layer Connection" Protocols            9  Address Resolution and the TCP/IP Address Resolution Protocol (ARP)                 9  Address Resolution Concepts and Issues

The Need For Address Resolution 1 2 3 Dynamic Address Resolution

Direct Mapping Not Possible With Large Hardware Addresses

Unfortunately, direct mapping only works when it is possible to express the data link layer address as a function of the network layer address. Consider instead the same IP address, 222.101.33.29, running on an Ethernet network. Here, the data link layer addresses are “hard-wired” into the hardware itself (they can sometimes be overridden but usually this is not done). More importantly, the MAC address is 48 bits wide, not 8. This means the layer two address is bigger than the layer three address, and there is no way to do direct mapping, as Figure 46 illustrates.

In general, then, direct mapping is not possible when the layer three address is smaller than the layer two address. Consider that Ethernet is the most popular technology at layer two and uses a 48-bit address, and IP is the most popular technology at layer three and uses a 32-bit address. This is one reason why direct mapping is a technique that is not only not widely used, but that most people don't know about!

The Need For Address Resolution 1 2 3 Dynamic Address Resolution