firewall - Router port forwarding stopped working

I have cable internet service through Charter Communications and feed two desktop computers through a Linksys RP614v3 router. One system is my wife's running WinXP Home Edition and the other is mine, running Vista Home Premium (sp1). I have port forwarding configured in the Linksys so I can access the Vista system remotely using TightVNC. Initially, it worked great and I was able to remotely tend email and access local files while out of town for work.

Lately, the cable internet service appears to flicker intermittently and upon return, my Mailwasher program loses ability to access the net and I've been unable to make the remote connection. When I reset the port forwarded for email in the router control panel, Mailwasher functionality returns but as I'm home when that happens, I have no easy way to check remote access until the next time I'm on the road or at work.

I'm at my wit's end -- the TightVNC client accesses fine from my wife's system from behind the modem/router setup but I don't know how to maintain whatever gets reset when I fiddle with the control panel and the need to do so at all is new. I accessed it fine for a week off and on while out of town a month ago and now I can't leave home and access it from work an hour later.

First you have to determine whether your "Cable Modem" has a firewall.

To circumvent connection issues when consumers plugged in a new computer without power cycling the modem, Cable Companies pretty much made a 1 port modem/router with a firewall. So consumers could just swap ethernet cables. Check the WAN ip address of the router and make sure it's something different than a 192.168.X.X.

If not, you have to log into the modems web interface (192.168.1.1 or whatever yours is) and turn off the firewall or forward all unsolicited traffic to the router.

I've seen this in westell modems, global link modems, cox cable modems. It's worth a shot.

Have you tried changing the port over which it runs? Perhaps Charter is blocking the normal VNC port?

Have you checked your port forwarding rules to be sure you are forwarding to the right IP addresses? You may have gotten a different IP from your DHCP server.

I would configure your router to use a non-standard port for the port forwarding. I know ISP (Cox specifically) block commonly used ports (25 mail, 80 web, etc) from being used. However, from your other comments, this doesn't seem to be the case, but it wouldn't hurt.

Additionally, I would set up your machine to have static internal IP or reserved DHCP address ssigned to it by the router. This way, you can be reasonably sure that you won't have to reconfigure your internal network constantly.

Also, you may want to be sure that your cable modem is getting the same external IP address from your ISP. With Cox, my external IP changes constantly, and changes drastically from 24.x.x.x to 79.x.x.x to 98.x.x.x. I've seen it change a few times in just a few weeks, followed by periods where it's pretty stable for months, then start changing constantly again.

if you find that it keeps changing, then you can use a service like dyndns.org to have a constant url to reach your site each time the address changes.

Not sure if you fixed it by now. My linksys was having a problem keeping a port open when I used single port fowarding, they fixed it in a firmware update. However the fix before that was to use range port fowarding and use the same number for both fields in the range.