Free Lab Guide Explains How Traceroute Uses TTL and ICMP to Map Network Hops

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A free, hands-on networking tutorial published by Protocol Lab walks learners through how the traceroute tool discovers each router along a network path. The mechanism relies on the IP Time-To-Live field, which every router decrements by one, triggering an ICMP time-exceeded message when it hits zero. By sending successive probes with increasing TTL values, traceroute identifies each hop in order until the destination is reached. The lab simulates a four-node chain — a client, two Linux routers, and a server — using Docker and containerlab so learners can observe the process with live packet captures. All lab materials, including topologies, configurations, and scripts, are freely available on GitHub.

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