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.
Routers also reply to solicitation messages they receive.
They may reply directly to the soliciting host, or they may wait a short random interval and reply with a multicast. Hosts may send solicitation messages when they start until they receive a response. Typically, three solicitation messages are sent at 3-second intervals. Alternatively a host may wait for periodic advertisements. Each time a host receives an advertisement, it updates its default router if the new advertisement has one with a higher preference value and sets the TTL timer for the entry to match the value in the advertisement.