On Ethernet, a Router connects two different Ethernet networks. As an Ethernet network is identified by the Net-ID (= part of the IP address), the Net-ID can be used to decide whether the network subscriber is in the same network or whether data packets must be transferred to a Router to pass on.
If necessary, a Router can convert the physical type of the network (e.g. convert from Ethernet to ISDN) but it cannot convert the protocol.
