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In a Port Forwarding guide I have read I should set port forwarding on my ADSL modem and disable its firewall. Until I did both of this, my torrent client was not visible as a server from outside. However, I am unsure what implications disabling firewall has and why it is needed.

Can anyone explain this?

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This guide refers to settings for a specific ZyXel router. Turning off the "Active Firewall" feature doesn't mean you have no protection. This setting provides security features beyond a 'basic' firewall.

The fact that you're doing port forwarding in the first place suggests that your inside network is running on non-Internet routable adddress space (e.g. such as 10.10.10.x or 192.168.x.x). This is pretty much the default setup of most home networks. All of your Internet traffic is natted to a routable address assigned to you by your ISP. Under normal circumstances this means that no connections can be initiated from the Internet at large into your network without you specifically allowing for it.

By setting up port forwarding, that's what you're doing - you're allowing that specific port in, and mapping it specifically to your PC. This is a normal setup and a perfectly acceptable way to accomplish your goal.

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By disabling the firewall you just physically connected your little network to well over 50,000 unknown other networks and all of their users.

Unless your are running an OS with no security flaws (there is not one) and no software with security flaws (like a BitTorrent), you need a firewall of some sort.

Go here and test your system to see for your self. www.grc.com

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Disabling your modem's firewall is not a big issue IF you have a software firewall enabled. Software firewalls are more flexible and updateable. You either already have one with your OS, or can easily get a free/paid solution.

You do not want your system exposed to the world (unless you welcome strangers looking up your private data and turning your machine into a bot for their scams/hacks/extortion schemes).

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