|| hping is a command-line oriented TCP/IP packet assembler/analyzer. The interface is inspired to the ping(8) unix command, but hping isnt only able to send ICMP echo requests. It supports TCP, UDP, ICMP and RAW-IP protocols, has a traceroute mode, the ability to send files between a covered channel, and many other features.
Using hping, you can: test firewall rules, perform [spoofed] port scanning, test net performance using different protocols, packet size, TOS (type of service), and fragmentation, do path MTU discovery, tranfer files (even between really Fascist firewall rules), perform traceroute-like actions under different protocols, fingerprint remote OSs, audit a TCP/IP stack, etc. hping is a good tool for learning TCP/IP.