The essential idea behind anycast is that there is a group of IPv6 nodes providing the same service. If you use an anycast address to identify this group, the request will be delivered to its nearest member using standard network mechanisms.

An anycast address is hard to distinguish. There is no separate part of the address space dedicated for these addresses, they are living in the unicast space. The local configuration is responsible for identification of anycast addresses.

Common routing methods should be used to maintain the information about the nearest anycast group member. It means that routers inside the network part containing the whole group should have a dedicated entry for this anycast address in their routing tables. This is a serious drawback for largescale anycast deployment, where the anycast group members are spread around the global Internet. Every such anycast address involves a separate entry in global routing tables. It contradicts one of the essential IPv6 addressing design principles: hold the global routing tables as small as possible.

But this is not the sole problem of anycast addresses. Another problem is found in the heart of the anycasting mechanism. Selection of a particular receiver of an anycast-addressed datagram is left to IP, which means it is stateless. In consequence the receiver can change during running communication which can be truly confusing for the transport and application layers.

Yet another problem is related to security. How do we protect anycast groups from intruders falsely declaring themselves to be holders of given anycast addresses and stealing the data or sending false responses. It is extremely inadvisable to use IPSec to secure anycast addressed traffic (it would require all group members to use the same security parameters).

There is a research focused on solving the anycast problem. Until it succeeds, there are serious limitations to the anycast usage. Especially:

• Anycast addresses must not be used as a sender address in the IP datagram.
• Anycast addresses may be assigned to routers only. Anycast-addressed hosts are prohibited.

In summary: anycast addresses represent an experimental area where many aspects are still researched.They are already deployed in limited scope - for example anycast address is used when a mobile node looks for home agent. The scope of this address is limited to its home network only, which makes anycast perfectly usable for such application. RFC 2526 [RFC2526] defines reserved IPv6 anycast addresses with the group span restricted to single subnet.