Internetworks are created by connecting together individual networks. The devices that are responsible for this connection of networks are routers, which send data from one network to the next. A host must rely on a router to forward transmissions to all devices other than those on the local network. For this reason, before a host can properly use an internetwork, it needs to find a local router and learn important information about both the router and the network itself. Enabling this information exchange is one of the most important jobs of the IPv6 Neighbor Discovery protocol.

The general term used to describe most of the ND communication between hosts and routers on a local network is discovery. As we discussed earlier in this section, the term encompasses not merely discovery of the router but also communication of important parameters. Most of this communication flows from the routers to the hosts, since routers really control the way that each network is used. They provide information to hosts so the hosts know how best to operate.

The various discovery features related to host-router communication are all facilitated by the same exchange of two different ICMPv6 message types. Router Advertisement messages are sent only by routers, and contain information about the router and also the network on which it is located. Router Solicitation messages are optional, and sent by hosts when they want to find a local router. The format of each of these messages is described in the section on ICMPv6 informational messages.