How to separate the network into different subnets

Internally, from computer with address 192.168.1.Y i can access device, ofcourse.

You haven't given us enough detail:

You have two separate logical networks

• 192.168.0.0/24
• 192.168.1.0/24

Your firewall lives in 192.168.0.100. This means that 192.168.1.0 cannot communicate with your firewall without some other part of the picture that I've missed. How does 192.168.1.0/24 communicate with your firewall? Your must either have:

• a secondary interface on the m0n0wall which also has an address on 192.168.1.0
• a router in between the two networks
• have a network bridge or some device doing NAT between the two networks

You'll need to fill us in on what part of the puzzle we're missing here.

You have no way to get the packets to the machine doing the routing. Machines on each network need to know that the router's interface on their network is their gateway to the other network.

The simplest way to do this is to add a static route to the default gateway on each network. For example, if the router is 192.168.1.10/24 and 192.168.2.20/24, and the default gateways for the two networks are 192.168.1.1 and 192.168.2.1, then you need to add the following two static routes:

1) On 192.168.1.1, you need a route to 192.168.2.0/24 with a gateway of 192.168.1.10

2) On 192.168.2.1, you need a route to 192.168.1.0/24 with a gateway of 192.168.2.20

That way, when machines on either network send packets for the other network to their default gateway, the default gateway will know to send the packets to the router that connects the two networks.