hub with a switch in our example, we get the setup
shown in Figure 17-8, where a Layer 2 switch maintains a
port-to-MAC address lookup table.
Figure 17-8 Using a Layer 2 Switch for Port-to-MAC
Address Mapping
As you can see in Figure 17-8, each of the devices is
“seen,” and the MAC address is registered when the first
packet is sent by the device. Over a period, the switch
generates a table that contains the port number on which
each MAC address was seen. It then uses this table to do
a lookup for the destination MAC address and sends the
incoming packet to the correct outgoing port. In the
figure, a packet destined for Device 3 is sent only to the
correct port.
Recall our look at the OSI model layers in Chapter 16.
The switch described so far in this section is a Layer 2
switch, which works at Layer 2 of the OSI model (the
data link layer). The switch sends a packet to a
destination port by using the MAC address lookup table,
which stores the MAC address of a device associated with
each port. In contrast, a Layer 3 switch works at the
network layer of the OSI model (Layer 3), which means it
looks at the IP address in order to do the switching.