I originally posted this at http://bdgregg.blogspot.com/2005/10/network-monitoring-network-often-gets.html.
The network often gets blamed for performance issues in Solaris. How busy are your network interfaces? How do you find out?
Many sysadmins are using "netstat -i" and looking at packet counts. Packet counts turn out to be not as useful as they seem – you don't know if they are big packets or small packets, so you really don't how utilised the network interface is.
I've written an website on network monitoring, to cover how to discover what is really happening on your network. To do this I use a variety to tools, including some based on Kstat and DTrace.