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I want to open one of my computer's ports to the outside world so that others can access it. 

   my computer                             gateway
@@@@@@@@@@@@@@@@@@                    @@@@@@@@@@@@@@@@@@
@                @                    @       (C)      @
@                @          ssh       @                @
@ my app <----- 1234 <==============> @ <------------ 8080 <--- curl gateway:8080
@                @ (A)    username    @                @ (B)
@   ssh client   @          (D)       @   ssh server   @
@@@@@@@@@@@@@@@@@@                    @@@@@@@@@@@@@@@@@@

Example

My application runs on my computer and I want people from outside of my network to access it.

Important

I'm sure that 'GatewayPorts' is set to 'yes' on the gateway.
If this is not the case, it can be set by executing this command on the gateway:

echo 'GatewayPorts yes' >> sshd_config