IPv6 - Addressing Modes
IPv6 - Addressing Modes
Section titled “IPv6 - Addressing Modes”Addressing modes define how IP packets are delivered to their destinations. IPv6 supports three primary addressing modes: Unicast, Multicast, and Anycast.
Unicast
Section titled “Unicast”Unicast is a one-to-one communication method. An IPv6 unicast address uniquely identifies a single interface on an IPv6-enabled device. A packet sent to a unicast address is delivered only to the interface identified by that address.
For example, when you browse a website, your computer (client) sends unicast packets to the web server’s unicast IP address, and the server sends unicast packets back to your computer’s unicast IP address. This is the most common form of communication on the internet.
Diagrammatically: A single source host sends a packet directly to a single, specific destination host.
Multicast
Section titled “Multicast”Multicast is a one-to-many communication method. An IPv6 multicast address identifies a group of interfaces, potentially on different devices. A packet sent to a multicast address is delivered to all interfaces that have joined that multicast group.
Hosts interested in receiving multicast traffic for a particular group must subscribe to that group using protocols like Multicast Listener Discovery (MLD). Examples of IPv6 multicast usage include streaming video to multiple subscribers or distributing routing protocol updates (e.g., OSPFv3 uses FF02::5 and FF02::6).
Well-known link-local multicast addresses include:
- FF02::1: All-nodes address (all IPv6 devices on the local link).
- FF02::2: All-routers address (all IPv6 routers on the local link).
Diagrammatically: A single source host sends a packet that is then delivered by the network to multiple specific destination hosts that are members of the target multicast group.
Anycast
Section titled “Anycast”Anycast is a one-to-nearest (in routing terms) communication method. An IPv6 anycast address is assigned to multiple interfaces, typically on different devices. A packet sent to an anycast address is routed by the network to only one of these interfaces—the one that is ‘closest’ or ‘best’ as determined by the routing protocols.
Anycast is often used for services like DNS root servers or content delivery networks (CDNs). Multiple servers around the world can share the same anycast IP address. When a user tries to access the service, their request is routed to the server topologically nearest to them, improving response time and providing load distribution and redundancy.
For example, if multiple DNS servers (e.g., in New York, London, and Tokyo) are configured with the same anycast IPv6 address, a user in Europe trying to resolve a domain name would likely have their DNS query routed to the London server, as it would have the lowest routing cost from their location.
Diagrammatically: A single source host sends a packet to an anycast address. The network then routes this packet to the ‘closest’ of several servers that share this same anycast address.