When you enable the Microsoft Built-In firewall, you usually cannot map drives to computers on your local network. The reason is because the purpose of the firewall is to block all of the ports that Windows normally uses to communicate with other machines on your network. You can resolve this. Unfortunately you have to open one of the more dangerous ports. This somewhat defeats the purpose of using the firewall in the first place.
The best solution is to create a VPN connection with the other machine so that you can connect securely through the VPN ports. I personally find this to be somewhat inconvenient. For whatever reason VPN connections to machines inside my network are buggy and intermitant – definitely not reliable for things such as mapping a network backup drive.
If you’re willing to trade a little security for convenience, then you can simply open port 137 to UDP traffic. Security experts would probably do a spit-take upon hearing this advice. Anyway, to do this, you go to the dialog where you enable the MS firewall and click on the “Settings” button. Click “Add” to open a new port. You can name it whatever you want. The target machine is your own machine, so you can enter “localhost” here. Specify the port is UDP and set both the internal and external value to “137″
One you open this port, Windows can use it to communicate normally with other machines on the network. Unfortunately, it opens your computer up a little (perhaps a lot) more as well. XP SP2 has a more advanced firewall where you can enable access on a per-application level. I installed a beta version of SP2 and i did like this feature, so i’m looking forward to the final release later this year.
