I'm using a RUTX08 FW RUTX_R_00.02.01.1 for a test setup.
DHCP (given IP: 192.168.1.6/24)
Portforewarding from any IP in WAN, TCP port 80 to 172.16.1.3:80 (a webserver, that works fine).
If I type in my browser (client from WAN side, IP:192.168.1.7/24) http://192.168.1.6 I get a "unreachable".
If I set up the WAN to other subnet address (never mind what) its working.
Many thanks for reading this threat.