Netstat

netstat displays network statistics. The most popular usage is to display your current routing table:

$ netstat -rn

If I wanted to see how many bytes flow through my interface and see this at an interval of 1 second I would type: $ netstat -I pppoe0 -w 1 pppoe0 in         pppoe0 out          total in           total out bytes              bytes             bytes               bytes 1396887118          221757068        2813821922           475500976           16620                 888             33492                2168

One can also see what existing connections/sockets are open:

$ netstat -na