In order to find a router, a node can send a Router Solicitation
to which a router will reply with a Router Advertisement
(RA). In addition to answering solicitations, the router also
can send this advertisement periodically, for example toupdate certain information. This Router Advertisement
contains among other information the network prefix which
the nodes must use for their IPv6 addresses, the lifetime of the
router which indicates the period in which the nodes may
consider the router valid and a preference level, which can be
used to order the preference of the router in the nodes. This
information is used by the nodes in the network to create an
address and to build the routing table.