Wireshark is the world's foremost network protocol analyzer. It lets you see what's happening on your network at a microscopic level.
More about Wireshark for Mac
It is the de facto (and often de jure) standard across many industries and educational institutions. Wireshark development thrives thanks to the contributions of networking experts across the globe. It is the continuation of a project that started in 1998. Wireshark was written by networking experts around the world, and is an example of the power of open source. Wireshark is used by network professionals around the world for analysis, troubleshooting, software and protocol development and education. The program has all of the standard features you would expect in a protocol analyzer, and several features not seen in any other product. Its open source license allows talented experts in the networking community to add enhancements. Learn more.