These devices also provide an interface between the Layer 2 device and
the physical media. Some common examples are a NIC installed in a host, bridge, or
switch. Figure 1-57 shows an example of Layer 2 devices.
Figure 1-57 Layer 2 Devices
Layer 1
Physical
A B C D
1 2 3 4
Layer 2
Data Link
OR
1 2
86 Chapter 1: Building a Simple Network
Layer 2 Addressing
Host communications require a Layer 2 address. Figure 1-58 shows an example of a MAC
address for a Layer 2 Ethernet frame.
Figure 1-58 Ethernet MAC Address
When the host-to-host communications were ?¬?rst developed, several network layer
protocols were called network operating systems (NOS). Early NOS were NetWare, IP,
ISO, and Banyan-Vines. It became apparent that a need for a Layer 2 address that was
independent of the NOS existed, so the MAC address was created.
MAC addresses are assigned to end devices such as hosts. In most cases, Layer 2 network
devices such as bridges and switches are not assigned a MAC address. However, in some
special cases, switches might be assigned an address.
Layer 3 Devices and Their Functions
The network layer provides connectivity and path selection between two host systems that
might be located on geographically separated networks.
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