Understand IPv6 addresses

Tuesday, April 7, 2009


IPv4 addresses are represented in dotted-decimal format.
32-bit address is divided along 8-bit boundaries.
Each set of 8 bits is converted to its decimal equivalent and separated by periods.

For IPv6, the 128-bit address is divided along 16-bit boundaries
Each 16-bit block is converted to a 4-digit hexadecimal number and separated by colons.
IPv6 Unicast:

This is similar to the unicast address in IPv4 – a single address identifying a single interface.

There are four types of unicast addresses:

Global unicast addresses, which are conventional, publicly routable address, just like conventional IPv4 publicly routable addresses.
Link-local addresses are akin to the private, non-routable addresses in IPv4 (,, They are not meant to be routed, but confined to a single network segment.
Unique local addresses are also meant for private addressing, with the addition of being unique, so that joining two subnets does not cause address collisions.
Special addresses are loopback addresses, IPv4-address mapped spaces

Multicast in IPv6 is similar to the old IPv4 broadcast address a packet sent to a multicast address is delivered to every interface in a group. The IPv6 difference is it’s targeted instead of annoying every single host on the segment with broadcast blather, only hosts who are members of the multicast group receive the multicast packets. IPv6 multicast is routable, and routers will not forward multicast packets unless there are members of the multicast groups to forward the packets to.


An anycast address is a single address assigned to multiple nodes. A packet sent to an anycast address is then delivered to the first available node. This is a slick way to provide both load-balancing and automatic failover. The idea of anycast has been around for a long time; it was proposed for inclusion in IPv4 but it never happened.

Several of the DNS root servers use a router-based anycast implementation, which is really a shared unicast addressing scheme. (While there are only thirteen authoritative root server names, the total number of actual servers is considerably larger, and they are spread all over the globe.) The same IP address is assigned to multiple interfaces, and then multiple routing tables entries are needed to move everything along.

IPv6 anycast addresses contain fields that identify them as anycast, so all you need to do is configure your network interfaces appropriately. The IPv6 protocol itself takes care of getting the packets to their final destinations. It’s a lot simpler to administer than shared unicast addressing.


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