IPv6 Network - IPv6 Addressing

IPv6 has three types of addresses: unicast, multicast, and anycast. Unicast addresses are regular addresses used for one-to-one communication. Multicast addresses are “group addresses”; packets sent to such an address are delivered to all the systems that are interested and have joined the group. All functions that were performed by broadcasts in IPv4 are performed by using multicasts in IPv6. This type has the advantage that systems that aren’t interested in certain information aren’t forced to spend CPU cycles receiving it anyway, like they are with broadcasts. With multicasts, the network interface ignores packets addressed to groups that weren’t joined at the hardware level. Anycasts are similar to multicasts, the difference being that packets sent to an anycast address are only delivered to one system in the anycast group rather than all of them.

The special address types in the ::/3 include two special addresses (discussed later this chapter), IPv6-mapped IPv4 addresses (discussed in Chapter 6), and IPv6-encoded NSAP/ CLNP addresses and IPX addresses. Link-local addresses are for use on a single subnet; they are discussed later this chapter. In a similar vein, site-local addresses are meant for use within a single site. The site-local address range is somewhat similar to the RFC 1918 address ranges in IPv4 (10.0.0.0/8,
172.16.0.0/12, and 192.168.0.0/16). However, the IETF has identified a number of concerns regarding the use of site-local addresses. See Chapter 4 for a more detailed discussion. Missing from Table 2-1 are anycast addresses, because anycast addresses are “syntactically indistinguishable” from unicast addresses. In other words, anycast addresses look the same as unicast addresses and share the same address space, and a host has no way of knowing whether it’s sending a packet to a regular unicast address or to an anycast group address. A system that is set up to receive anycast packets must be explicitly configured so it knows it’s dealing with an anycast address in order to enable the required special link layer behavior.

• Caution Multicast and anycast addresses may be used as destination addresses in packets, but only unicast addresses may be used as source addresses. Also, only routers may be configured with an IPv6 anycast address.

This means strictly speaking, anycasting services such as the DNS isn’t compatible with RFC 3513. After all, for anycast DNS to work, the anycast address must be present on more than one DNS server (which presumably are hosts and not routers), and the responses to DNS queries are sent back with the destination address of the query (the anycast address) as the source address. The host that sent the query wouldn’t recognize the response if it came from a different address. However, anycasting services is rarely done using the actual IPv6 anycast link-level mechanism, where several systems configured with the same anycast address are connected to the same subnet.

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