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Cooper Bussmann BU-245U-E Wireless Ethernet & Device Server User Manual
3.13 Compatibility

3 Address (Layer 3 Bridge) & 4 Address Mode

What Addresses are in a wireless Ethernet data frame?
There are two different Wi-Fi communication “modes”,3 Address and 4 Address Modes. Each mode has a slightly different way that it addresses
the data frames to other devices on the network.
In 3 Address Mode the addressing is made up of a Destination Address (DA),a Source Address (SA), and a BSSID (MAC Address of the Access
Point)
In 4 Address Mode the addressing is made of a Destination Address (DA),a Source Address (SA), a Radio Transmit Address (RTA),and a Radio
Receive Address (RRA).
Packets between AP and Client
Communications between Access Points and Client is generally done using 3 Address Mode because there are only 3 addresses within the
communication path as indicated in the path from the Laptop to Station #1 in the diagram below.
If the Laptop wishes to communicate to Station #1 the DA will be Station #1, the SA will be the Laptop and the BSSID will be the APs MAC
Address hence the 3 Address Mode.
The example shows the Laptop may need to communicate to the
Ethernet Device connected to Station #2.
You can see that there is now a fourth address that cannot be
addressed using the 3 Address Mode of communications.
i.e., Laptop,AP, Station #2 and Ethernet Device.
The example shows the Access Point and the Station as both being
BU-245U-E modules which will happily communicate as both support
4 Address Mode.
However,there are some instances where a third Party Access Point
may not be able to communicate using 4 Address Mode to a Client
(Sta).
There are only a couple of ways around this sort of situation.
• Changing Station #2 to a Router, however this may mean that Station #2 and the Ethernet Device will use a different network address scheme
which may be inconvenient and will require more setup.
• Configure the Access Point to communicate 4 Address Mode
If choosing the later this will only be possible if the Station and the AP support 4 Address Mode,some third Party APs do not.
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