The Network Layer insures that the transmission gets to the proper network segment. Above the Data-Link layer, so must deal with something other than Frames and MAC addresses. The Network Layer and Real Protocols. IP - Internet Protocol. IPX - Internet Packet Exchange.
Routers can link different networks by linking to other routers.
Many routing strategies are possible. We will not discuss them all. RIP - Routing Information Protocol has been the most common strategy. Since each Network Protocol is different, a router may need to understand more than one. Single Protocol Router.