Routing 192.168.0.0/16 into a 192.168.0.0/24 network
Routing from a 192.168.0.0/16 (LAN port, gateway at 192.168.1.1, DHCP from 192.168.1.100 to 192.168.255.254) into a WAN 192.168.0.0/24 gateway at 192.168.0.1, with other things connected at 192.168.0.2…
is it possible to go from a LAN port (lets say my ip is 192.168.1.100) to the "outside" at 192.168.0.2??? How? (can't touch the outside network config, and I need to have a 192.168.0.0 / 16 on the inside)
Not without specific routes for 192.168.0.0/24 on every device that's on 192.168.0.0/16. Even then I would consider the configuration, umm, creative.
In theory the longest subnet mask should win when routing decisions are made. I wouldn't expect every client device to behave properly, however.
Someone should renumber. I would start with the person who made the brilliant decision to use 192.168.0.0/16. No possibility of a routing collision there. Not with probably 10s of millions of networks on 192.168.0.0/24 or 192.168.1.0/24. And since it's all DHCP it should be relatively easy.
I guess it might also be possible to do a 1:1 NAT for the entire 192.168.0.0/24 network so, to your LAN, it appears as something that should be sent to the default gateway.
even if you do, somehow, manage to get this working - it'll be a nightmare; if not now, then later.
bite the bitter pill and go to a sane subnetting scheme
my 2 cents