The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Control Message Protocol (ICMP/ICMPv4 and ICMPv6)                 9  ICMP Message Types and Formats                      9  ICMP Version 4 (ICMPv4) Informational Message Types and Formats

ICMPv4 Timestamp (Request) and Timestamp Reply Messages 1 2 3 4 ICMPv4 Address Mask Request and Reply Messages

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 (224.0.0.1), since they are intended for hosts to hear, while Router Solicitation messages use the “all routers” multicast address (224.0.0.2). 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 and OSPF.

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 network.

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.

ICMPv4 Timestamp (Request) and Timestamp Reply Messages 1 2 3 4 ICMPv4 Address Mask Request and Reply Messages