IPv6 Network Administration

Packets and Structures

The IPv6 packet structure is, in most ways, very similar to the IPv4 packet structure. Some fields have been removed and some have been added, but the most obvious change is the size of the addresses. While the IPv4 source and destination addresses are 32 bits each, IPv6 addresses are 128 bits each. The reason for 128 bits is discussed in the following sidebar, Is 128 Bits Enough?

Is 128 Bits Enough?

People who remember the second law of thermodynamics (or who have worked in large organizations) know that it is impossible to have a perfectly efficient system. As it is with heat exchangers, so it is with network protocols.

In particular, when addressing endpoints in a real network, from any limited pool, there is a certain amount of the addresses that will be "lost," or inefficiently allocated. This is not simply due to factors in the protocol itself (for example, wasting addresses on the broadcast address in point to point links) but is also due to real world concerns like administrative error, customer churn, aggregation, and so on. It turns out that this error is actually measurable, and we do it using a metric called the host density ratio, or HD ratio. This is a number that increases from zero to one as the address space fills. For reference, it's defined as:

Empirical calculations for telephone number allocation and network address assignment show that a