If I am using a VPN and my friend is using the same VPN and has some program listening on a specific port on his computer, can I access that computer and port with a local IP address instead of having to use his real ip address and configure port forwarding?
I.E. can I access it just like we are on the same home network?
The answer depends on where the VPN is "terminating". There are two possibilities.
If the VPN is terminated at the PC, e.g. in Windows, then no you cannot (or rather should not) since traffic to the remote network is forced into the VPN on the computer & you would have to define a "split tunnel" to hive off some traffic to go to other devices on the local network without it going through the VPN. That is insecure and adds significant risk to the remote end of the VPN, not recommended, at least without specific permission from the owner of the VPN.
If the VPN is terminated at the router, your local network is an extension of the remote network and then you can talk between local PC's.
It is possible that you have a VPN only to hide some activity from your local service provider or local law enforcement. In that case, creating a split tunnel is probably not really an issue. But to give specifics would require further information about the OS being used and the VPN software and configuration.
With a VPN, you extend your private network over untrusted public network (the Internet).
You should then be able to reach other computers with their local IP address, as soon as your client is configured for and if your network topology is compatible : you can be on different subnets for instance.
