If you understand half of following links, you have a good start.Read them through and google its terminology.Some of these files are really old, but it's good enough to have a glimpse of idea what you are getting into.http://netfilter.org/documentation/HOWTO/networking-concepts-HOWTO.htmlhttp://www.faqs.org/docs/Linux-HOWTO/Net-HOWTO.htmlhttp://tldp.org/HOWTO/VPN-HOWTO/https://help.ubuntu.com/community/IptablesHowTohttp://dag.wieers.com/howto/ssh-http-tunneling/http://tldp.org/HOWTO/Bridge+Firewall.htmlhttp://tldp.org/HOWTO/ATM-Linux-HOWTO/index.htmlhttp://tldp.org/HOWTO/Apache-WebDAV-LDAP-HOWTO/index.htmlhttp://tldp.org/HOWTO/Intranet-Server-HOWTO.htmlhttp://tldp.org/HOWTO/DNS-HOWTO.html