AT-8024 and AT-8024GB Installation Guide
37
A Few Basics to Ethernet Switching
An Ethernet switch interconnects network devices, such as workstations,
printers, routers, and other Ethernet switches, so that they can
communicate with each other by sending and receiving Ethernet
frames.
MAC Address
Table
Every hardware device on your network has a unique MAC address. This
address is assigned to the device by the device’s manufacturer. For
example, when you install a network interface card (NIC) in a computer
so that you can connect it to the network, the NIC already has a MAC
address assigned to it by its manufacturer.
The AT-8024 and AT-8024GB Fast Ethernet Switches contain a 4 kilobyte
MAC address table. The switch uses the table to store the MAC addresses
of the network nodes connected to its ports, along with the port number
on which each address was learned.
The switch learns the MAC addresses of the end nodes by examining the
source address of each packet received on a port. It adds the address
and port on which the packet was received to the MAC table if the
address had not already been entered in the table. The result is a table
that contains all the MAC addresses of the devices that are connected to
the switch’s ports, and the port number where each address was
learned.
When the switch receives a packet, it also examines the destination
address and, by referring to its MAC address table, determines the port
on which the destination node is connected. It then forwards the packet
to the appropriate port and on to the end node. This increases network
bandwidth by limiting each frame to the appropriate port when the
intended end node is located, freeing the other switch ports for
receiving and transmitting data.
If the switch receives a packet with a destination address that is not in
the MAC address table, it floods the packet to all the ports on the switch.
If the ports have been grouped into virtual LANs, the switch floods the
packet only to those ports which belong to the same VLAN as the port
on which the packet was received. This prevents packets from being
forwarded onto inappropriate LAN segments, increasing network
security. When the destination node responds, the switch adds its MAC
address and port number to the table.
If the switch receives a packet with a destination address that is on the
same port on which the packet was received, it discards the packet
without forwarding it on to any port. Since both the source node and the
destination node for the packet are located on the same port on the
switch, there is no reason for the switch to forward the packet.
