Forcing your IP is only useful if you set up your router to do port-forwarding. If you don't do that or if you do that incorrectly, forcing your IP will achieve nothing.

On the contrary, other clients may have difficulties to identify you as firewalled. They will try to connect to you directly and might see more failed downloads. - Especially other firewalled users will try to download from you although such a download attempt can never be successful.

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Morgens ess ich Cornflakes und abends ess ich Brot
Und wenn ich lang genug gelebt hab, dann sterb ich und bin tot

--Fischmob