Before a host computer can send IP datagrams beyond its directly attached subnet, it must discover the address of at least one router on that subnet. This discovery can be accomplished using ICMP messages called Router Advertisements and Router Solicitations. An Advertisement message is sent by a router periodically to announce its presence and the addresses of its interfaces. When a computer connects to a network, rather than waiting to receive Router Advertisements, it can send Router Solicitations to request that routers send their advertisements immediately.