The number of entries in the message. The length of an entry in 32-bit units. The number of seconds that an entry will can be considered valid. A signed 32-bit level indicating the preference to be assigned to this address when selecting a default router for a subnet. Each router on a subnet is responsible for advertising its own preference level.
Larger values imply higher preference, smaller values imply lower.
The default is zero, which is in the middle of the possible range. The ICMP header code field is zero for both of these messages. These two messages are used if a host or a router supports the Router Discovery Protocol. The use of multicasting is recommended, but broadcasting may be used if multicasting is not supported on an interface. Routers periodically advertise their IP addresses on those subnets where they are configured to do so.