I have a network with the following structure.
The server in PC2 is simple socket server TCP in 8080 port.
I need to access to PC2 from other external network by socket client. This socket client knows the public IP (85.xxx.xxx.x), the private IP (192.168.0.21) and the port.
How I can access PC2 without port forwarding on the router?
If possible, I would like to have a reference in any programming language of this case.
How I can access PC2 without port forwarding on the router?
Assuming a NAT router (which is almost certain given a 192.168.0.0/24 internal network), you can't do this without port-forwarding at the router. Or something very similar.
For example if you can SSH to the router and then to the server you could use SSH tunelling (which is a form of port-forwarding).
You could also have PC2 SSH to a public server and make available a tunelled endpoint there. (you might need something like netcat or socat to assist with this)
In general, port-forwarding at the router is the way to go.
There are ways to enable this sort of communication without manual configuration of port-forwarding but they usually rely on router support and/or an external connection broker.
General
Specific
I would like to have a reference in any programming language of this case
If you are writing the software at both ends there are probably many examples you can follow.
For example, see
