DCC, short for Direct Client-to-Client is an IRC-related sub-protocol enabling users to directly connect using an IRC server. Once established, a typical DCC session runs independently from the IRC server between the 2 clients. Used for Chat and file sharing, DCC allows users privacy independent of the IRC Server they are connected to.

DCC connections can be initiated in two different ways.
The most common way to initiate a DCC session is use CTCP. The CTCP is sent from one user, over the IRC network, to another. Another is for the client to connect directly to the DCC server. Using this method, no traffic goes across the IRC network therefore the users involved do not need to be connected to an IRC network in order to use DCC.
SDCC, short for Secure Direct Client-to-Client, also known as DCC SCHAT is a variation of the DCC protocol allowing two users to chat privately over IRC with encryption.

Please Note

There is a DCC SEND IRC related bug in some Netgear, Linksys, and d-link routers. Replacing the [character argument] with a string of 14 or more characters, not including spaces will cause users to forcefully disconnect if they are affected. This is likely due to the smart fire walling feature of many routers. Disabling it may resolve the issue. This exploit is confined to port 6667, so a temporary fix should include setting your client to connect to a different port.

Using mIRC DCC

Your mIRC client will need to be setup properly to use DCC. By default direct connections are blocked by nearly every firewall and router. The key to successful use of DCC is properly setting up all hardware and software. You will need to gather the information needed prior to setup, which will include the port range, your inhouse ip, and possibly additional settings for any Software Firewall, or Security Suite you may be using.

mIRC uses a port range of 1024-5000. The mIRC client allows the user to select as many or as little ports as they choose. It is recommended that no less then 10 ports be opened for DCC Chat, and no less the 25 ports for DCC Sends.

Access mIRC Advanced port settings by selecting mIRC options. Expand Connect, select Options, select Advanced. Select DCC and enter the ports you wish to use. Select Use Random Ports. Using Bind sockets to IP is optional.

Preview the settings here.

Please Note. If you haven't already setup Identd and selected server in setup options do so now.

Setting Up Your Router

Now that your mIRC client is properly setup to use DCC you'll need to allow those same ports in your router setup. While there are many brands of routers, most utilize the same basic setup options. Your particular router will have come with a booklet explaining how to enter the program setup using your web browser.

Once you've entered setup, locate Port Forwarding. This is usually found under the Advanced Tab.
You will need to create 2 rules for your mIRC client.
The first is Ident which uses port 113 to identify your client to the IRC server.
The second will be the port range you choose to open for your DCC connections, remember the mIRC port range is 1024-5000. Choose the amount of consecutive ports you wish to open.
Select TCP for the connection type.
Enter your inhouse IP and Enable.
Save Router Settings.

Note. Inhouse IP refers to the IP the router has assigned your computer. To find your inhouse IP, Select your Operating System Instructions below.

Windows 98 Users type, winipcfg and hit enter.
Windows XP Users type, ipconfig /all
Windows Vista Users have 2 ways to access the IP address.

Using Network and Sharing Center

From the Start Menu, right-click on Network and select Properties. Select View status to the right of your Local Area Connection. In the Local Area Connection Status window that opens, select Details. Your IP Address will be listed among the other connection details.

Using the Command Prompt

From the Start Menu, type Command Prompt in the search field then click Command Prompt once it appears type ipconfig and press Enter.

Windows Firewall

The Windows Firewall is a source of basic security. It is not recommended to disable it. It is advised you enter the Windows Firewall setup and open the ports you have chosen in your previous setups. Since the Windows firewall does not allow port ranges you will need to set a rule for each port you wish to open.
If you choose to disable the Windows Firewall, please note that simply turning it off does not disable it. You must disable the service that starts it. Once you have done so you must restart your computer for the connection to be released from the firewall rules.

Additional Software Settings

The above settings should allow DCC without issue, however many users have software firewalls and security suites installed that will also block DCC connections. Many of these require a simple setting giving the mIRC client full permissions, known as a trusted application. While I can't possibly cover every security program available this tutorial can be used for general information as a starting point for DCC troubleshooting.

Additional Notes

Once your sure you've completed the above steps, it is recommended that you completely shut down your network. Begin by shutting down your computer, turn off your router and modem. Wait several minutes and boot in this order, router, modem and your computer. Test your settings.