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The following TCP ports are used to provide common network services such as e-mail and remote access.
In the interest of security, a person would normally only forward secure ports to an inside host. For example, you might forward port 995 (POPS) instead of port 110 (POP) as the standard POP protocol passes passwords across the Internet in plain text, whereas POPS is an (SSL) encrypted protocol.
These are some of the most common ports that might be to be forwarded to and internal server from an external 'gateway/router/firewall' device.
You should only forward the ports for the services you want to expose to the outside (Internet).