Router: multiple networks - 1 port. Maybe?

Question

Computer networks

There are 4 PCs. We divide them manually into 2 networks, 2 PCs each, and connect them to the switch. The switch is connected to the router.
Question: the router must have an ip of each network, but it is connected to the switch through one port. In this case, it is impossible to organize communication between networks?

Answer 1

If the router allows you to assign several ip to one port, then I don’t see any problems. Another thing is that the overwhelming majority of soho routers cannot do this. And so, a microtic for 20 bucks will do it once or twice.

Answer 2

Maybe.
Option 1, beautiful: if the switch can VLAN, split it into two VLANs.
PCs are connected to access ports.
The switch port facing the router is configured as a trunk, and both VLANs are registered in it.
The port on the router is also configured as a trunk with the same VLANs.
Option 2, scary: see the answer from fshp
But then the whole point of splitting into two subnets is lost: the Ethernet domain is the same!
And very smart MS Windows, with such a switching scheme, manages to communicate with neighbors living in a different subnet on the switch _directly_, and not through a router.
It turns out interesting: Windows, having met an unfamiliar ip-address, _always_ sends an ARP request first.
And if he receives an answer (and he will receive it: there is only one Ethernet domain!), then instead of the ip-routing table, he will use the ARP table, and traffic between nodes from different networks will go _only_ through the switch, without taxiing along the way to router.
In general, VLAN and VLAN only)

Answer 3

"It turns out to be interesting: when Windows encounters an unfamiliar IP address, it _always_ sends an ARP request first."
I also thought so, that's why I asked the question: "they say, why don't computers from different subnets communicate using the Arp table" Here is a link to this question: Why can't computers on different networks communicate?
The answer turned out to be simple: for an ip address from a different subnet, the ARP request is not even initialized.