Sudden problem with LAN access to some of the net

[Claude Jones]

This problem has so stumped me I don't know if I can fairly describe it, but I 
post what I can here to see if it strikes a chord with anyone else. My 
scenario is an FC4 machine that is right on the net - I use this as my home 
machine and it is mostly for experimentation and email. I use FWBuilder to 
protect it. It serves as dhcp server to several pc's in my home all running 
Windows. All has been working well for months. Two nights ago, I was trying 
to get bittorrent to work and was experimenting with opening various ports. 
That same night I did some updates. That's a poor description, I realize, but 
it is just to set the stage. The next morning, two of the pc's on my Lan had 
lost some of their connectivity. My wife's laptop, for example, couldn't get 
email or run messenger - just returned 'couldn't connect' messages. However, 
she could still browse websites. Similar problems with a second pc that I 
don't have much access to, so I can't be specific but, he couldn't get mail. 
Two other pc's continued to function normally. I fired up an old laptop I 
keep here for testing, and it connected and all functions worked. 

So -- dhcp is working - the problem machines are getting valid addresses - 
they continue to have some functionality
I suspect an issue with DNS - I had set up a DNS server on my FC4 box months 
ago, and it has been working well - dhcp gives out 127.0.0.1 as the dns 
server to the lan machines - what I did to fix the problem was to change that 
entry in dhcp to my isp's DNS server - presto, things started working...

So, any ideas on what I could have broken with my DNS configuration? 
-- 
Claude Jones
Bluemont, VA, USA