In addition to many commercial tools such as EtherPeek Network Analyzer, WildPackets has provided some of the best free tools. I've always been a fan of IP Subnet Calculator™ myself. Check out some of their other free tools:
My favorite is netdoppler. Great tool to measure throughput, latency etc., really comfortable. pathchar for Linux/Unix offers roughly the same functionality but is somewhat outdated and difficult to understand as there is no real documentation.
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