Netstat is a simple and great network utility.
Here are a few useful usage examples:
What Network Services are Running?
netstat -atup
or
netstat -ap | grep LISTEN | less
This can be helpful to determine the services running.
Need stats on dropped UDP packets?
netstat -s -u
or TCP
netstat -s -t
or summary of everything
netstat -s
or looking for error rates on the interface?
netstat -i
Listening interfaces?
netstat -l