Hello @NikoMax ,
NAT has been almost exclusive feature of routers and firewalls ( first case was Cat6500 with Sup720 for NAT support).
traffic from switch 1 is sent to the router over a specific port and VLAN this may map to a router subinterface on the router.
The NAT action is triggered when traffic entering a L3 interface or subinterface with ip nat inside configured has to go out another L3 interface or subinterface with ip nat outside.
When this happens the router performs a translation ( usually of IP source only) of the IP source either to an address taken from a NAT pool or by using PAT aka NAT overload and in this last case using as new source IP address the IP address of the outgoing interface.
The translated traffic is destination based routed by the router and it may come back to the same Switch1 but on a different VLAN and/or port ( again associated to a different VLAN to the same port is that port is a L2 trunk on switch side and the router has multiple VLAN based subinterfaces this is called router on a stick to perform inter VLAN routing).
This should help you understand the flow of traffic in your network
To be noted the router keeps tracks of translation sessions that is performing and it is able and it has to perform un NAT on return traffic translating the destination address ( the so called Outside Global) to the private IP address ( Inside ) that triggered the NAT session. So two ways communications via a NAT performing device is possible.
You can use
show ip nat translation
on the router to check NAT activity .
other useful commands can be
show ip nat statistics.
Hope to help
Giuseppe
