Traceroute exploits a fundamental property of TCP/IP; it uses the ICMP type 11 message to determine each router's IP address by sending packets that have low TTL values. When a data packet is transmitted, it contains a TTL value, which gets decremented by each router that handles it; a TTL value of 0 results in that packet not being forwarded, and a ICMP type 11 packet being returned to the sender. By sending the first packet with a TTL value of 1, the IP address of the first router will be repeated. Send the next packet with a TTL value of 2, and the second router's IP address will be known. Repeat this process until a packet reaches the target, and you will have traced the route.