As widely installed as ping, but less well-known, is traceroute ("tracert" under Windows). This shows all the routers that data passes through on its way to a specified destination, passed in as the argument (e.g. traceroute www.cisco.com).

netstat

Once you have connectivity issues out of the way, netstat (network statistics) shows all active connections, both incoming and outgoing as well as other useful pieces of information.

Firstly, running netstat by itself lists hosts that are presently communicating with your computer and the port they are using. Couple this with the -p flag (-b on Windows) to see the name of the local process (or binary file) being used to perform the communication. You should see expected entries, like e-mail programs but you may also discover surprising processes that are chewing up bandwidth.

Next, run netstat with the -r flag; this gives entirely different output and shows the local computer's routing tables. This helps understand where the computer is sending packets to, based on the destination IP address.

There are many other useful statistics to be gained from netstat; it can give extended information. It can show Ethernet statistics. It can give figures for each network interface. It can print a one-off list or it can continue running, refreshing values each second. Unfortunately, the way in which these options are invoked - as well as how the results are formatted - varies from implementation to implementation. It is worth becoming familiar with the man page for your local flavour of netstat but always keep it in mind as an essential tool that shows the paths your computer uses for data transfer as well as all the network activity running on your machine right now.