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

Address Resolution Concepts and Issues 1 2 3 Address Resolution Through Direct Mapping

General Address Resolution Methods

In fact, not only do we need to have way of making this translation, we need to be concerned with the manner in which it is done. Since the translation occurs for each hop of every datagram sent over an internetwork, the efficiency of the process is extremely important. We don't want to use a resolution method that takes a lot of network resources.

Address resolution can be accomplished in two basic ways:

• Direct Mapping: A formula is used to map the higher-layer address into the lower-layer address. This is the simpler and more efficient technique but has some limitations, especially regarding the size of the data link layer address compared to the network layer address.
  
  
• Dynamic Resolution: A special protocol is used that allows a device with only an IP address to determine the corresponding data link layer address, even if they take completely different forms. This is normally done by interrogating one or more other devices on a local network to determine what data link layer address corresponds to a given IP address. This is more complex and less efficient than direct mapping but is more flexible.

The next two topics explore these two methods in more detail.

You should bear in mind that of necessity, it is not possible to have a fully general address resolution method that works automatically. Since it deals with linking data link layer addresses to network layer addresses, the implementation must be specific to the technologies used in each of these layers. The only method that could really be considered generic would be the use of static, manually-updated tables that say “link this layer three address to this layer two address”. This, of course, is not automatic and brings with it all the limitations of manual configuration.

Address Resolution Concepts and Issues 1 2 3 Address Resolution Through Direct Mapping