I would never trust any IP address in the private ranges. You might have someone elses network
You are assuming that the server has two NICs, one on a private LAN, and another on the Internet connections. This is a rare configuration. Also, it is fairly hard to determine which interface the connection came from. You can look at the incoming IP address. It isn't that reliable for scurity.
It is better to look at the remote IP address. This can be spoofed and isn't very good security. But if you are looking at the IP address for trust, you aren't interested in good security.
