just send a packet (any type) to the destination, with its time-to-live (ttl) field set to 1 first. this packet will be dropped by the first router, which will send us an icmp error message, telling us that the time-to-live has exceeded. that is how we get the ip address of the first router. we then increase our ttl field to 2, send out another packet, and get the ip address of the second router. we will repeat this procedure until our packet finally reach the destination.