See, There are several no of network monitoring sw in the market. No one can be guaranteed u this is the best ! Various people have various opinions. Though u didn’t mention here what OS you are using. I am providing an in-built tool for Server 2003
- Open Add/Remove Programs in the Control Panel
- Click the Add/Remove Windows Components button
- Select Management and Monitoring Tools, and then click Details
- In this window, select the Network Monitor Tools check box, and then click OK
- Click Next, if prompted for additional files insert ur Windows 2003 Server CD or type the appropriate network path to these files
- Click finish with installation is complete
Top: ManageEngine OpManager
- iptraf (You have to download from here. It’s free
- Nagios (You have to download from here)
Hope this may help you!
@ PrabhuVairak provided the link, the SW is not free, when I tried to download it indicates for 30 days trial version. Would u like to mention here what OS you are using! Because, SW are different for various OS. And hope, you know it very well.
And also Carlosdl provided a very good link here! You have to look it also!