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ICMPv4 Router Advertisement and Router Solicitation Messages
(Page 4 of 4)
Addressing and Use of Router Advertisement and Router Solicitation Messages
If possible, both Router Advertisement
and Router Solicitation messages are sent out multicast for efficiency.
Router Advertisements use the all devices multicast
address (18.104.22.168), since they are intended for hosts to hear, while
Router Solicitation messages use the all routers
multicast address (22.214.171.124). If the local network does not supports
multicast, messages are instead sent out broadcast (to address 255.255.255.255).
It is important to remember that
just like ICMP
Redirect messages, Router Advertisement
messages are not a generalized method for exchanging routing information.
They are a support mechanism only, used to inform hosts about the existence
of routers. Detailed information about routes is communicated between
routers using routing
protocols, like RIP
I should also mention that while
I suggested router discovery was the alternative to manual configuration
of a host's default router, there are other alternatives as well. For
example, a host configuration protocol like DHCP
can allow a host to learn the address of a default router on the local
Finally, note that when Mobile
IP is implemented, Router Advertisement
messages are used as the basis for Mobile-IP-aware routers to send Agent
Advertisements. One or more special extensions are added
to the regular Router Advertisement format to create an Agent
Advertisement. This is discussed extensively in the
topic on Mobile IP agent discovery.
Key Concept: ICMP Router Advertisement messages are sent regularly by IP routers to inform hosts of their presence and characteristics, so hosts know to use them for delivery of datagrams to distant hosts. A host that is new to a network and wants to find out immediately what routers are present may send a Router Solicitation, which will prompt listening routers to send out Router Advertisements.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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