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When a host on the network boots, and the
host has not been manually configured with a default gateway, it can
learn of available routers through the process of router discovery.
This process begins with the host sending a router solicitation
message to all routers, using the multicast address 224.0.0.2 as the
destination address. Figure
 shows the
ICMP router discovery message. The router discovery message may also
be broadcast to include routers that may not be configured for
multicasting. If a router discovery message is sent to a router that
does not support the discovery process, the solicitation will go
unanswered.
When a router that supports the
discovery process receives the router discovery message , a router
advertisement is sent in return. The router advertisement frame format
is shown in Figure
and an
explanation of each field is shown in Figure
 .
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