So far, we've covered the topics required to discuss how packets of data get routed from computer A to host B across LANs and/or WANs over distances that may range from across a room to across the globe. The important concepts to remember are that Ethernet frames work with switches and IP packets work with routers to accomplish this feat, which we'll cover in the next section.
To reiterate what was outlined in the layer 2 (the data-link layer) discussion, Ethernet frames are switched from the entry port to the appropriate destination port based on the destination MAC address. Network switches build tables of which MAC addresses belong to each port, compare a frame's destination MAC address to these tables, and switch the frame to the appropriate egress port if the destination is on the same switch or out a trunk port to another switch or router otherwise.
Note that the first time a switch sees a destination MAC address it doesn't recognize...