Software based
It might be software based.
Microsoft services are mostly background apps. One is called ICF, Internet Connection Firewall. Most likely it was enabled during the setup of your network if you've set it up so that one computer is acting as a router/NAT. I agree that everyone should use a firewall; just not Microsoft's.
To check if its enabled or not:
1. Go to Settings > Network Connections
2. Right-click on your Internet connection and select Properties
3. Go to the Advanced Tab
The settings for ICS and ICF should be right in front of you. If it's enabled, just disable ICF. Or if insist on using Microsoft's firewall, click on Settings and continue to read:
4. Here you will see which services you can enable. You have your basic services such running a FTP server, POP/SMTP server, Telnet Server, Web server, whatever server. If you want to add a different kind of service click add.
In your case you will want to check FTP server. Hit 'OK'. Hit 'OK' in the Properties dialog and you are all done.
I hope that worked for you. This article also applies to anything such as 425 connections or basically anything if you have a firewall already installed and you are wondering what the heck is going on after you've permitted the specific events.
Cheers,
Lytener