4sysops - FREE: Wireshark - Powerful network analyzer

Wireshark is the world’s foremost network protocol analyzer, and is the de facto (and often de jure) standard across many industries and educational institutions.

This is the modest description of its publisher. Wireshark, formerly called Ethereal, indeed has a long tradition and is certainly the most famous network packet sniffer. Administrators usually need such a tool for network troubleshooting. Wireshark supports hundreds of network protocols. Using its powerful filters, you can watch in detail how two applications communicate. The recently released new version 0.99.7 closes some severe security holes.

WireShark

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I compiled a short list of other free protocol analyzers some time ago. A newcomer in the league of sophisticated network sniffers is Microsoft’s Network Monitor.

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