RFC1918, an Internet standards document, describes several non-routable IP address blocks. These blocks of IP addresses have been set aside for use by individuals and companies for use in private networks. Properly configured routers on the Internet will not route these IP addresses.

In short, if your local computer's IP is in any of these ranges, you will very likely need to configure your router and firewall correctly to operate DC++ in active mode.

10.0.0.0 through 10.255.255.255 (10/8 prefix)
172.16.0.0 through 172.31.255.255 (172.16/12 prefix)
192.168.0.0 through 192.168.255.255 (192.168/16 prefix)