Wireshark is a great tool for capturing and analyzing packets for network troubleshooting, analysis, software and communication protocol development, education and reverse engineering. It is a free and open-source software that was originally known as Ethereal. It can run on various OS platforms like Microsoft Windows, Linux, OS X, Solaris, FreeBSD, NetBSD, OpenBSD and many others.

If you are familiar with tcpdump, Wireshark is very much similar to it only that it comes with a graphical user interface which greatly simplify your task in analyzing the network packet.