Hello Konrad
I’ll do my best to clarify as much as possible for you!!
The role of a router is to receive IP packets and route them from one subnet to another based on routing rules that exist within the router (statically assigned routes or routes learned from routing protocols such as OSPF or EIGRP). As such, a router will have multiple layer 3 interfaces, each one assigned an IP address on a different subnet. It will receive a packet on one interface and route it out another. Router interfaces cannot be assigned IP addresses in the same subnet. If you attempt to configure two interfaces on the same subnet, it will give you an error. Such a configuration defeats the purpose of the router, which is to route packets from one subnet to another.
Layer 2 switches have switchports, or Layer 2 ports. These don’t have IP addresses and cannot be assigned IP addresses, because a layer 2 switch cannot route packets.
Now on a layer 3 switch, it is possible to configure what is known as a “routed port”. When configured, such a port would behave exactly the same as a port on a router, with an assigned IP address in a unique subnet (unique compared to other IP addresses assigned to interfaces on the local device).
The types of interfaces depend upon the type of connections an enterprise needs. Is it serial, fiber, xDSL, Ethernet, or wireless? What technology is being used at that location? Depending upon this, you can procure the appropriate router with the right interfaces. Now as is the case with all routers, as we mentioned, each interface must have an IP address in a different subnet.
I hope this has been helpful!
Laz