Physical - Layer 1 Transmits data over a physical medium. Defines cables, cards and physical aspects. Manages data placement on and data removal from the network media. This was done as part of a transition from a U.S. government Internet over to a commercial Internet. Common Network Components Gateways Translates between one or more layers of the OSI model.
Routers Can connect LANs and form WANs.
Determines the path along which network traffic should be sent. They are devices that can filter and forward packets between LAN segments. A Switch forwards frames only to the appropriate destinations device's port. Know what device works at what layer. Reserved IP Numbers ranges are for Private use and should not be advertised on the Internet - You could use NAT to translate between you Private and Public ranges.