I just read an interesting article in the Dutch C'T magazine about Portknocking:

Because a lot of WL-500g users have services running on the WAN side, security is an issue for them. While most home users won't be a major target for hackers, one should do as much as possible to prevent hackers accessing the router and LAN clients.

Compromised networks can be used by hackers to store illegal or criminal material on your computer. While this doesn't sound very serious, it really is: the need for a good security has recently been been emphasised by a court verdict on someone who had a few pictures of nude children on its computer which he hadn't downloaded on perpose. This means you can be convicted on posession of illegal/criminal material even if you're not aware of having this material on your computer.

The method of Portknocking provides a radical new way of securing your network. It can be accomplished by using the iptables in an ingenious way in such a way that your network seems to be completely stealthed to the outside world without cripling your services to the WAN.

If you've become intersted in Portknocking you can read all about it here: www.portknocking.org