Most of which I'm about to type & copy/paste you probably already know but... Also not sure if you have to enter the entire IP address into the app./gaming/dmz field or just the last 3 IP numbers for your router? The image below looks like you have to enter the whole thing? Mine I just need to enter the last 3.
If you set DMZ for an IP that doesn't always mean the IP for the device hasn't changed. If you turn off the power to that device, your router and/or modem the IP's might switch around depending on in what order you turn them on, or it recognizes the device(s). EG: If my PC is .100 I turn it off, power the router off and on again, then turn on my PS3 the PS3 will take .100. So the DMZ set for PC .100 is now PS3 .100. So I have to cmd.exe/ipconfig, get the new IP and change the DMZ in router settings.
To enable/change DMZ settings you have to log into your router. (192.168.0.1 into a web browser) Enter the name/pasword you chose while installing and setting up security for the router.
From the cisco DCP3825 manual:
Applications & Gaming > DMZ
Use this page to configure an IP address whose ports are directly exposed to the public Internet or to the Wide Area Network (WAN). Demilitarized Zone (DMZ) hosting is commonly referred to as "exposed host," and allows you to specify a recipient of WAN traffic that Network Address Translation (NAT) is unable to translate to a known local PC.
A DMZ is typically used by a company that wants to host its own Internet server. DMZ allows one IP address to be placed on the Internet side of the gateway firewall while others remain protected behind the firewall.
The DMZ allows a device to be directly accessible to Internet traffic, such as a web (HTTP) server, an FTP server, an SMTP (e-mail) server, and a domain name system (DNS) server. Select the DMZ tab to open the Applications & Gaming DMZ page.
Applications and Gaming DMZ Page Description
Use the descriptions and instructions in the following table to configure the port range triggering for the residential gateway. Select enable for each DMZ Host IP address. After you make your selections, click Save Settings to apply your changes or Cancel Changes to cancel.
DMZ Hosting
Select the desired option:
· Enable
· Disable (factory default)
DMZ Host IP Address
DMZ allows one IP address to be unprotected while others remain protected. Enter the IP address of the computer you want to expose to the Internet in this field.
Hope this helps.