How to Port Forward for 'Garry's Mod'

Log in to your router using its Internet protocol (IP) address, which varies based on the manufacturer. Linksys access is '192.168.1.1' with the default username blank and password of 'admin.' NetGear uses 'admin' for the username and either 'password' or '1234' by default.
Click the 'Advanced' tab, then select 'Port Forwarding.' It may also be located under 'Applications and Gaming.'
Type the name of the application, 'Garry's Mod,' into the application.
Enter '27005' for the Start and '27015' for the End range.
Select 'Both' from the protocol drop-down menu. Check the box next to 'Enable' to turn it on.
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