IPv6 Addressing Formats Beyond the Basics

Chapter 3, Introducing IPv6 (Overview) introduces the most common IPv6 addressing formats: unicast site address and link-local address. This section includes in-depth explanations of addressing formats that are not covered in detail in Chapter 3, Introducing IPv6 (Overview):

6to4-Derived Addresses
IPv6 Multicast Addresses in Depth

6to4-Derived Addresses

If you plan to configure a 6to4 tunnel from a router or host endpoint, you must advertise the 6to4 site prefix in the /etc/inet/ndpd.conf file on the endpoint system. For an introduction and tasks for configuring 6to4 tunnels, refer to How to Configure a 6to4 Tunnel.

The next figure shows the parts of a 6to4 site prefix.

Figure 11-1 Parts of a 6to4 Site Prefix

image:This figure shows the format of a 6to4 site prefix and shows a site prefix example. The cited tables explain the information in the figure.

The next figure shows the parts of a subnet prefix for a 6to4 site, such as you would include in the ndpd.conf file.

Figure 11-2 Parts of a 6to4 Subnet Prefix

image:This figure shows the format of a 6to4 prefix and shows a prefix example. The following context explains the information in the figure.

This table explains the parts of a 6to4 subnet prefix, their respective lengths, and their definitions.

Part	Length	Definition
Prefix	16 bits	6to4 prefix label 2002 (0x2002).
IPv4 address	32 bits	Unique IPv4 address that is already configured on the 6to4 interface. For the advertisement, you specify the hexadecimal representation of the IPv4 address, rather than the IPv4 dotted-decimal representation.
Subnet ID	16 bits	Subnet ID, which must be a value that is unique for the link at your 6to4 site.

6to4-Derived Addressing on a Host

When an IPv6 host receives the 6to4-derived prefix by way of a router advertisement, the host automatically reconfigures a 6to4-derived address on an interface. The address has the following format:

prefix:IPv4-address:subnet-ID:interface-ID/64

The output from the ifconfig -a command on a host with a 6to4 interface might resemble the following:

qfe1:3: flags=2180841<UP,RUNNING,MULTICAST,ADDRCONF,ROUTER,IPv6>
 mtu 1500 index 7
        inet6 2002:8192:56bb:9258:a00:20ff:fea9:4521/64

In this output, the 6to4-derived address follows inet6.

This table explains the parts of the 6to4-derived address, their lengths and the information they provide.

Address Part	Length	Definition
`prefix`	16 bits	`2002`, which is the 6to4 prefix
`IPv4-address`	32 bits	`8192:56bb`, which is the IPv4 address, in hexadecimal notation, for the 6to4 pseudo-interface that is configured on the 6to4 router
subnet-ID	16 bits	`9258`, which is the address of the subnet of which this host is a member
interface-ID	64 bits	`a00:20ff:fea9:4521`, which is the interface ID of the host interface that is configured for 6to4

IPv6 Multicast Addresses in Depth

The IPv6 multicast address provides a method for distributing identical information or services to a defined group of interfaces, called the multicast group. Typically, the interfaces of the multicast group are on different nodes. An interface can belong to any number of multicast groups. Packets sent to the multicast address go to all members of the multicast group. For example, one use of multicast addresses is for broadcasting information, similar to the capability of the IPv4 broadcast address.

The following table shows the format of the multicast address.

Table 11-1 IPv6 Multicast Address Format

8 bits	4 bits	4 bits	8 bits	8 bits	64 bits	32 bits
`11111111`	`FLGS`	`SCOP`	`Reserved`	`Plen`	`Network prefix`	`Group ID`

The following is a summary of the contents of each field.

11111111 – Identifies the address as a multicast address.
FLGS – Set of the four flags 0,0,P,T. The first two flags must be zero. The P field has one of the following values:
- 0 = Multicast address that is not assigned based on the network prefix
- 1 = Multicast address that is assigned based on the network prefix
If P is set to 1, then T must also be 1.
Reserved - Reserved value of zero.
Plen - Number of bits in the site prefix that identify the subnet, for a multicast address that is assigned based on a site prefix.
Group ID - Identifier for the multicast group, either permanent or dynamic.

For complete details about the multicast format, refer to RFC 3306, "Unicast-Prefix-based IPv6 Multicast Addresses.

Some IPv6 multicast addresses are permanently assigned by the Internet Assigned Numbers Authority (IANA). Some examples are the All Nodes Multicast Addresses and All Routers Multicast Addresses that are required by all IPv6 hosts and IPv6 routers. IPv6 multicast addresses can also be dynamically allocated. For more information about the proper use of multicast addresses and groups, see RFC 3307, "Allocation Guidelines for IPv6 Multicast Addresses".

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