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| 2.1 |  | Using a Layered Architectural Model to Describe Data Flow |
| 2.1.3 |  | Network devices utilize control information |

Layer 2 network devices utilize the control information within a frame to assess where a frame is physically destined to on a local network segment. The physical address, or MAC address, of the destination network adapter, or interface, is read so that the proper decision on switching to an appropriate port can be made. In addition to addressing information, the Layer 2 device can check on the validity of the frame by recalculating the frame check sequence (FCS) and matching it with the FCS included as part of the encapsulation process at the data-link layer.

Layer 3 network devices are responsible for determining logical paths between networks through an internetwork. Layer 3 devices read the networking address of a destination contained within the control information of packets, and then forward them to an appropriate interface. Layer 3 addressing is hierarchical so that intermediate devices need only know which network the destination device is a member of in order to deliver the packet to the correct location.

Data flow alternates between the physical medium which is stage two of data flow, and Layer 2 and 3 devices representing the third stage in the flow of data from a source to a target end-system.