Each IP packet has a Time to Live (TTL) section that keeps track of the number of network devices the packet has passed through to reach its destination. The server sending the packet sets the initial TTL value, and each network device that the packet passes through then reduces this value by 1. If the TTL value reaches 0, the network device will discard the packet.
This mechanism helps to ensure that bad routing on the Internet won't cause packets to aimlessly loop around the network without being removed. TTLs therefore help to reduce the clogging of data circuits with unnecessary traffic.
Remember this concept as it will be helpful in understanding the traceroute troubleshooting technique outlined in Chapter 4, "Simple Network Troubleshooting", that covers Network Troubleshooting.