These resources provide further information on iptables/netfiter or are otherwise a resource for use while administering a firewall.
Netfilter/IPTables Home
http://www.netfilter.org/
RFC 1918 - Address Allocation for Private Internets
http://www.faqs.org/rfcs/rfc1918.html
IANA Assigned Port Numbers
http://www.iana.org/assignments/port-numbers
Ports for Internet Services - another port list
http://www.chebucto.ns.ca/~rakerman/port-table.html
Oskar Andreasson's IP-Tables Tutorial
http://www.linux-sxs.org/iptables/
Linux IP Masquerade HOWTO
http://www.tldp.org/HOWTO/IP-Masquerade-HOWTO/index.html
Firewalling with netfilter/iptables
http://www.knowplace.org/netfilter/
Linux 2.4 NAT HOWTO
http://netfilter.samba.org/unreliable-guides/NAT-HOWTO/index.html
Linux 2.4 Packet Filtering HOWTO
http://netfilter.samba.org/unreliable-guides/packet-filtering-HOWTO/index.html
IPTables Firewall Scripts and Configuration Files
http://www.malibyte.net/iptables/scripts/fwscripts.html
IPTables Connection Tracking
http://www.cs.princeton.edu/~jns/security/iptables/iptables_conntrack.html