The first packet has a time to live (TTL) value of one, the second two, and so on. When a packet passes through a host, normally the host decrements the TTL value by one, and forwards the packet to the next host. When a packet with a TTL of one reaches a host, the host discards the packet and sends an ICMP time exceeded (type 11) packet to the sender. The traceroute utility uses these returning packets to produce a list of hosts that the packets have traversed en route to the destination. traceroute may not list the real hosts, it indicates that the first host is at one hop, the second host at two hops. IP cannot guarantee that all the packets take the same route.