http://searchnetworking.techtarget.com/tip/How-to-use-traceroute-to-troubleshoot-network-problems

Learning how to use tracery is a key component of network trace back tools. Understanding how to use traceroute can help you identify the number of devices, hops, between you and the destination system. Traceroute functions by using the time-to-live (TTL) field in the IP header. Each router that touches an IP packet will decrease the TTL field by one.