I have a Mikrotik behind my main firewall. The eth1 is plugged on the main switch with subnet 192.168.166.0/24. The eht2 is connected to a switch in the building next door with subnet 192.168.165.0/24. Eth3 is connected to switch upstairs with subnet 192.168.164.0/24. There is communication between all the subnets but not internet acces for subnets 192.168.165.0/24 and 192.168.164.0/24. What am I missing?
Where’s your internet connection plugged in to? The best would be to have your internet connection on another port on the Mikrotik.
Make sure all subnets’ default gateway is the IP of the mikrotik port on their subnet.
Make sure you have a masquerade rule that include all source ranges, or 3 separate masquerade rules or a rule not specifying any source address.