IPv6 expanded routing and addressing
IPv6 increases the IP address size from 32 bits to 128 bits, thereby supporting more levels of addressing hierarchy, a much greater number of addressable nodes, and simpler autoconfiguration of addresses.
IPv6 has three types of addresses:
| Item | Description |
|---|---|
| unicast | A packet sent to a unicast address is delivered to the interface identified
by that address. A unicast address has a particular scope: link-local, site-local,
global. There are also two special unicast addresses:
|
| multicast | A packet sent to a multicast address is delivered to all interfaces identified by that address. A multicast address is identified by the prefix ff::/8. As with unicast addresses, multicast addresses have a similar scope: node-local, link-local, site-local, and organization-local. |
| anycast | An anycast address is an address that has a single sender, multiple
listeners, and only one responder (normally the "nearest" one, according to
the routing protocols' measure of distance). For example, several web servers
listening on an anycast address. When a request is sent to the anycast address,
only one responds. An anycast address is indistinguishable from a unicast address. A unicast address becomes an anycast address when more than one interface is configured with that address. |
IPv6 autoconfiguration
The primary mechanisms available that enable a node to start up and communicate with other nodes over an IPv4 network are hard-coding, BOOTP, and DHCP.
IPv6 introduces the concept of scope to IP addresses, one of which is link-local. This allows a host to construct a valid address from the predefined link-local prefix and its local identifier. This local identifier is typically derived from the medium access control (MAC) address of the interface to be configured. Using this address, the node can communicate with other hosts on the same subnet and, for a fully-isolated subnet, might not need any other address configuration.
IPv6 meaningful addresses
With IPv4, the only generally recognizable meaning in addresses are broadcast (typically all 1s or all 0s), and classes (for example, a class D is multicast). With IPv6, the prefix can be quickly examined to determine scope (for example, link-local), multicast versus unicast, and a mechanism of assignment (provider-based or geography-based).
Routing information might be explicitly loaded into the upper bits of addresses as well, but this has not yet been finalized by the IETF (for provider-based addresses, routing information is implicitly present in the address).
IPv6 duplicate address detection
When an interface is initialized or reinitialized, it uses autoconfiguration to tentatively associate a link-local address with that interface (the address is not yet assigned to that interface in the traditional sense). At this point, the interface joins the all-nodes and solicited-nodes multicast groups, and sends a neighbor discovery message to these groups. By using the multicast address, the node can determine whether that particular link-local address has been previously assigned, and choose an alternate address.
This eliminates accidentally assigning the same address to two different interfaces on the same link. (It is still possible to create duplicate global-scope addresses for nodes that are not on the same link.)
Neighbor discovery/stateless address autoconfiguration
Neighbor Discovery Protocol (NDP) for IPv6 is used by nodes (hosts and routers) to determine the link-layer addresses for neighbors known to reside on attached links, and maintain per-destination routing tables for active connections. IPv6 defines both a stateful and a stateless address autoconfiguration mechanism. Stateless autoconfiguration requires no manual configuration of hosts; minimal, if any, configuration of routers; and no additional servers.
Hosts also use NDP to find neighboring routers that are willing to forward packets on their behalf and detect changed link-layer addresses. NDP uses the Internet Control Message Protocol (ICMP) Version 6 with its own unique message types. In general terms, the IPv6 Neighbor Discovery protocol corresponds to a combination of the IPv4 Address Resolution Protocol (ARP), ICMP Router Discovery (RDISC), and ICMP Redirect (ICMPv4), but with many improvements over these IPv4 protocols.
The stateless mechanism allows a host to generate its own addresses using a combination of locally available information and information advertised by routers. Routers advertise prefixes that identify the subnets associated with a link, while hosts generate an interface token that uniquely identifies an interface on a subnet. An address is formed by combining the two. In the absence of routers, a host can only generate link-local addresses. However, link-local addresses are sufficient for allowing communication among nodes attached to the same link.