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IPv6

table from several locations, each client will use the ‘nearest’ server to source the content. It is worth noting that anycast addresses are taken from the global unicast address space and are syntactically indistinguishable from any other unicast address.

Anycast addresses in IPv6 require a prefix length, which defines the topological area within which all the members of the anycast group are located. Within that area, the anycast address must be maintained as a host route. If, as might commonly be the case, the devices are distributed globally, then there is no way to constrain the scope of the host route. This is clearly going to limit the scalability and applicability of such a mechanism.

11.4 STATELESS AUTOCONFIGURATION

Manual configuration of addresses represented as 32 hexadecimal digits on every host in a network certainly presents an administrative headache. Such long addresses are prone to misconfiguration through simple typing errors. However, in this area IPv6 provides significant potential benefits over IPv4. The term stateless refers to the fact that no server needs to maintain any state with respect to the available networks or the range of valid host addresses. Instead, the interface address is derived from information already held by the interface (e.g. the MAC address) and the network prefix announced by on-net routers in their router advertisements. The routers sourcing the router advertisements clearly know the prefix because they have a manually configured interface address with an associated prefix length. This does not provide any obvious benefit for routers in the network. However, for hosts, it provides a significant benefit. The administrative load associated with managing large numbers of hosts attached to a network is significantly reduced. There is, of course, nothing to prevent the address of an interface being statically configured or the use of a stateful autoconfiguration mechanism like DHCP.

11.5 DOMAIN NAME SYSTEM (DNS)

As mentioned above, addresses represented as 32 hexadecimal digits are prone to mistyping and also, significantly, are more difficult to remember than the dotted quad format of IPv4 addresses. Therefore, DNS takes on a new importance with respect to providing human readable addresses for IPv6 interfaces. Names are mapped to IPv6 addresses using AAAA (pronounced ‘quad A’) records. The domain analogous to IPv4’s in-addr.arpa for IPv6 is ip6.int. Reverse resolution of IPv6 addresses uses a very long format as demonstrated below:

1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.F.E.D.C.B.A.9.8.7.6.5.4.3.2.1.ip6.int.

This is the reverse entry for 1234:5678:9ABC:DEF0:0000:0000:0000:0001.