Installing, configuring and observing a single-machine firewall. Capturing and analyzing packets.
Writing a packet sniffer. Sniffing packets in the network, analyzing them. Capturing passwords from telnet connections this way.
Using programs such as traceroute, tcpdump, ping, etc.
Implementing a web server.
Implementing an FTP client.
Implementing a Telnet client.
Implementing a POP server and client.
Implementing a GNUtella client.
Installing (downloading, compiling and installing on all of the machines) the Network Simulator and simulating networks on it.