If you would like to get a deeper knowledge of how networking works, you should investigate the following:
TCP/IP Network Administration, Third Edition, by Craig Hunt (O'Reilly Media, Inc., 1992)
Virtual Private Networks, Second Edition, by Charlie Scott, et al. (O'Reilly Media, Inc., 1998)
Building Internet Firewalls, Second Edition, by Elizabeth D. Zwicky, et al. (O'Reilly Media, Inc., 2000)
Also, you can investigate the following manpages on your system:
ip
ifconfig
netstat
appletalk
ipfw