Why do subnets connected to the same router ping each other?

Question

Computer networks

Hello. I'm starting to study computer networks, I decided to practice. I sketched a diagram. Three departments on different subnets, connected to different router interfaces. Can you please tell me why these subnets ping each other?

Answer 1

Because
1) Routing is enabled on the Main Router
2) The gateway on the subnet of other departments is correctly configured on the computers of the departments.

Answer 2

Because the job of a router is - all of a sudden - to route! And when everything is configured correctly and there are no prohibiting rules, everything should work.

Answer 3

Even if you turn on a router instead of the current switch - why shouldn't the subnets ping each other if it is not configured (and by default it is) restrictive rules and vlans?

Answer 4

THOSE. Is it necessary to prescribe prohibiting rules (drop) for access from network to network in the router settings? Something like /ip firewall filter
add action=drop chain=forward dst-address=192.168.18.0/24 src-address=192.168.12.0/24
add action=drop chain=forward dst-address=192.168.12.0/24 src -address=192.168.18.0/24
Or the same in routes?
And if from a PC in one of the networks you need to access hosts from all networks, then how?