We have used the netstat program many times throughout the text. This program serves multiple purposes:

It shows the status of networking endpoints. We showed this in Section 5.6, when we followed the status of our endpoint as we started our client and server.
It shows the multicast groups that a host belongs to on each interface. The -ia flags are the normal way to show this, or the -g flag under Solaris 2.x.
It shows the per-protocol statistics with the -s option. We showed this in Section 8.13, when looking at the lack of flow control with UDP.
It displays the routing table with the -r option and the interface information with the -i option. We showed this in Section 1.9, where we used netstat to discover the topology of our network.

There are other uses of netstat and most vendors have added their own features. Check the man page on your system.