Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 149
IPv6 Addressing

The addressing of IPv6 is probably the best-known feature of IPv6. Ask anyone about the differences between IPv4 and IPv6, and the first response will be the change from 32-bit to 128-bit addressing. While it is true that the addressing was changed to 128 bits, there are many more features about the address space and its allocation that were carefully crafted. The first expansion of the address space was to 64 bits. It was later increased to 128 bits. It was proven that 64 bits was easily adequate but 128 bits was the final outcome. IPv6 addresses provide the same function as IPv4: identifiers for interfaces and sets of interfaces. Even though 128 bits are written for use, currently only 15 percent of the available address space is allocated for use. There are three types of addresses:

Unicast. An identifier for a single interface. A unique address delivered to a single destination.

Anycast. New for IP (version 6), an anycast address is an identifier for a set of interfaces (typically belonging to different nodes). This is similar to a multicast, but a packet sent to an anycast address is delivered to one of the interfaces identified by that address (the “nearest” one, according to the routing protocols’ measure of distance).

Multicast. An identifier for a set of interfaces (typically belonging to different nodes). A packet sent to a multicast address is delivered to all interfaces identified by that address.

In IPv6, a broadcast address is not defined. It was superseded by multicast addresses.

In IPv4, we identified addresses by their 32-bit value, normally, written in a form known as dotted-decimal notation; for example, 132.1.8.10. An IPv6 address is written in hexadecimal and consists of groupings of 8 containing 4 hexadecimal digits or 8 groups of 16 bits each. This takes the form:

xxxx : xxxx : xxxx : xxxx : xxxx : xxxx : xxxx : xxxx

FEDC:BA98:7654:3210:FEDC:BA98:7654:3210 is an example of an IPv6 address. Another example is the following unicast address:

1080:0000:0000:0000:0008:0800:200C:417A

Since writing an IPv6 address has become unwieldy (DNS becomes very important here), there are provisions allowed to condense the address into its smallest available form. Therefore, the preceding unicast address can be compressed into:

1080:0:0:0:8:800:200C:417A

or even

1080::8:800:200C:417A

The double colon has special significance. It is a demarcation point for compressing leading 0s. Notice here that only leading 0s can be compressed and the :: symbol can be used only once during the compression.

Therefore if you had the address of:

1080:0:0:5698:0:0:9887:1234

you cannot write it as:

1080::5698::9887:1234

The algorithm that runs the expansion for the address would get confused. How many 0s go in each of the colon-compressed slots? The correct way to compress this address would be to compress one or the other sides.

IPv6 Addressing (continued)

1080::5698:0:0:9887:1234 or 1080:0:0:5698:: 9887:1234164

As with IPv4, the first few bits of the IPv6 address tells something about the address (this has nothing to do with Class addressing). The table in Section 165 shows this. This time we are not dealing with classes or addresses, more so where the address has been assigned an address type and is known as the format prefix.

To get the amount of space used is simple. The formula is 1 / 2 ⁿ X, where x is the number of bits used. For example, if the first 8 bits are 0000 0000, then this is 1 / 2 ⁿ 8, or 1/256.

Prefixes are also used in this environment just like in the CIDR environment. A /30 indicates the first 30 bits are used for routing. Also notice that fields in certain types of addresses are given names to further identify the subaddress portions. Refer to slide 165 for an example.

There are three address types that are assigned out of the 0000 0000 format prefix space. These are the “unspecified address,” the loopback address, and the IPv6 addresses with embedded IPv4 addresses. This allocation supports the direct allocation of provider addresses, local use addresses, and multicast addresses. Space is reserved for NSAP addresses, IPX addresses, and geographic addresses. The remainder of the address space is unassigned for future use. This can be used for expansion of existing use (e.g., additional provider addresses, etc.) or new uses (e.g., separate locators and identifiers).

A value of FF (11111111) identifies an address as a multicast address; any other value identifies an address as a unicast address. Multicast addresses are used extensively throughout autoconfiguration of addresses and neighbor discovery. Anycast addresses are taken from the unicast address space, and are not syntactically distinguishable from unicast addresses.