I‘d like to inroduce 5 best network traffic monitoring tools to you.
Wireshark
Wireshark has always been one of the top monitors available. Wireshark is a cross-platform network monitor that does deep inspections of hundreds of protocols. It does live capture and capture save, which can be viewed in a few different modes. Wireshark also does VoIP analysis and can read/write many capture formats (tcpdump, Pcap NG, Microsoft Network Monitor, Cisco Secure IDS iplog, and many more).
http://www.wireshark.org/
Angry IP Scanner
Angry IP Scanner is one of the easiest to use of all the network traffic monitoring tools. It's interface is super easy to use and is a powerful little tool. One nice thing about Angry IP Scanner is that it is cross platform and doesn't require installation, so you can use it as a portable monitor. It can get NetBIOS information, favorite IP address range, Web server detection, customizable openers, and much more.
http://angryip.org/w/Download
Colasoft Capsa Free
This is what I'm using now. If you're an administrator for a Windows based network and are used to more Windows-like tools, Capsa Free might be the best tool for you. This software comes in both a free and paid form. The free version should be enough for most people. It provides an easy-to-use dashboard you can use to create various types of captures. The free version also offers plenty of alarm configurations so you can be alerted when something occurs. It can capture more than 400 network protocols, so you won't be missing out on anything with this free tool.
http://www.colasoft.com/capsa-free/
Zenmap
Zenmap is a graphical front end to the cross-platform Nmap tool. Nmap can scan huge networks, is free, is portable, and has great readme docs. It's one of the most powerful IP traffic monitors. However, due to this, it has quite the steap learning curve. Zenmap takes Nmap and makes it more accessible to users who prefer to avoid the command line. That does not mean Zenmap is the easiest of the lot. You still need to use some commands though which contributes to its tougher to use nature.
http://nmap.org/zenmap/
EtherApe
If you are a Linux user, then EtherApe is probably for you. It is a linux only tool that offers an easy-to-use mapping of IP traffic on your network. It does this in real time and gives you a clear picture of the overall look of your network traffic. You can create filters to make reading the map easier. EtherApe will display both the node and link color with the most-used protocol so it's easier to take a quick glance, even on a busy network.
http://etherape.sourceforge.net/
At this point, hopefully, you are all ready to monitor your network traffic for any problems.