Tools
http://ports.my-addr.com/tcp_port-udp_port-application-and-description.php
http://www.subnetonline.com/pages/subnet-calculators/ipv4-to-ipv6-converter.php
pf
iptables
http://serverfault.com/questions/244036/iptables-rules-to-allow-l2tp-ipsec-vpn-behind-firewall
Custom applications
iOS: Understanding Notifications
Links
http://netfilter.org/ (iptables)
www.linuxhomenetworking.com – Quick_HOWTO_:_Ch14_:_Linux_Firewalls_Using_iptables
http://lindesk.com/2007/04/internet-connection-sharing-using-iptables/
Linux Advanced Routing & Traffic Control HOWTO – Classifying packets with filters