Ethernet frames: what are the source and destination MAC addresses

There are actually 4 sets of Locally Administered Address Ranges that can be used on your network without fear of conflict, assuming no one else has assigned these on your network:

x2-xx-xx-xx-xx-xx
x6-xx-xx-xx-xx-xx
xA-xx-xx-xx-xx-xx
xE-xx-xx-xx-xx-xx

Replacing x with any hex value.

Not that I am in the business of defending Microsoft, but after reading through the following RFCs which attempt to define how broadcasts work, I don't think that Microsoft is necessarily violating any RFCs. IMO the problem should be fixed at the application level (i.e. directed broadcasts, not global) which will hit the appropriate routes in the routing table and only be sent from the correct interface for that IP network.

• RFC922
• RFC919

They both state that there is no standard defined for broadcasts. It also mentions in 919 that a specific physical interface should be selected for the broadcast. In the case of a multi-homed, multi-NIC machine generating the broadcast, I don't think that it's clearly stated what should happen. Broadcasts are never supposed to be passed by routers from one interface to the other, so is the Windows machine a router or not in this case?
If it is acting as a router, then any host responding to the broadcast with the incorrect IP address for that network (Adapters 2 and 3 in your example) should send the packet back to the ethernet address of Adapters 2 and 3 in response to Adapter 1's IP address and the Windows host should route it to the proper interface.
That sounds confusing... but can't think of a better way to phrase this

And finally, RFC 919 specifically says From RFC 919

Since we assume that the problem has already been solved at the data link layer, an IP host wishing to
send either a local broadcast or a directed broadcast need only
specify the appropriate destination address and send the datagram as
usual. Any sophisticated algorithms need only reside in gateways.

Reading that would suggest that the source IP address is irrelevant for a broadcast.


Since each application seems to handle broadcasts differently, I think that's where the responsibility resides. For example. nbtstat sends out directed broadcasts on multi-NICed machines, whereas games might use global broadcasts.
In short, the application should be fixed, not the OS in this case...

EDIT: Here is a link for the same circumstances, but on Linux. The linux kernel handles it by only sending one packet out the default interface (NIC A in this example). They recommend that the application enumerate the NICs and send a directed broadcast out each NIC. Link