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

Inflexibility of Direct Mapping

Now let’s consider the next generation, IP version 6? IPv6 supports massive 128-bit addresses. Furthermore, regular (unicast) addresses are even defined using a method that creates them from data link layer addresses using a special mapping. This would in theory allow IPv6 to use direct mapping for address resolution.

However, the decision was made to have IPv6 use dynamic resolution just as IPv4 does. One reason might be historical, since IPv4 uses dynamic resolution. However, the bigger reason is probably due to a disadvantage of direct mapping: its inflexibility. Dynamic resolution is a more generalized solution, because it allows data link layer and network layer addresses to be independent, and its disadvantages can be mostly neutralized through careful implementation, as we will see.

In fact, evidence for this can be seen in the fact that dynamic resolution of IP is in fact defined on ARCNet, the example we just used. We could do direct mapping there, but it restricts us to a certain pattern of IP addressing that reduces flexibility.

The Need For Address Resolution 1 2 3 Dynamic Address Resolution