Go to mIRC>OPTIONS>CONNECT>LOCALINFO ..... clear the Local Host and IP Address Fields> check IP Address and Server and click OK and reboot mIRC.
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DCC is behind a router
To use DCC behind a firewall, you need open the ports for it on your router and enter the port range into mIRC. Due to the large range of firewalls available we regret we care unable to offer support on how to open the ports in your router. There are many tutorials on the net on how to administrate your router's firmware interface. Once you have done this please enter go to mIRC>OPTIONS>ADVANCED..... and enter your port range in the fields highlighted in yellow . Make sure DCC is checked. Recommended portranges are for example ports from 60001 - 60010, which will allow you to make 10 concurrent dcc transfers at once. This portrange is not used by any other protocol, so it is not likely to interfere with anything, or pose a security threat as well. A very good starting-point for finding out how to configure your router is: http://en.wikipedia.org/wiki/Port_forwarding which has at the bottom of the page links to very helpful resources.
