relationship between ip and mac level boradcasts

[carl_townshend]

Hi all, can anyone tell me the relationship between the mac and ip layer when sending broadcasts? ie

does the mac address for any broadcast ie subnet level, all hosts etc stay the same, ffff.ffff.ffff ?

[Yudong Wu]

Give you an example:

R1--(e0)R2(E1)--10.1.1.0/24

When R1 sends IP broadcase to the network behind R2, the destination IP will be 10.1.1.255, destination MAC will be R2 e0's mac address.

When R2 receive it, R2 will change the destination mac to ffff.ffff.ffff and then sends it out to E1 interface.

All hosts in subnet 10.1.1.0/24 will process the packet with destination mac ffff.ffff.ffff.

HTH

[carl_townshend]

So, is any broadcast ie, 255.255.255.255, or 10.1.255.255 , does the mac address for this always use ffff.ffff.ffff ?

and also, i thought routers dont allow directed broadcasts ?

[Yudong Wu]

Yes, the last hop router will check the destination mac to ffff.ffff.ffff.

255.255.255.255 will broadcast to all IP networks.

IP directed broadcasts can be enabled by "ip directed-broadcast" command under the interface. (should be disabled by default) But, it's not secure to enable it.

[carl_townshend]

hi there

so do all ip broadcasts use the mac address ffff.ffff.ffff ?

[Yudong Wu]

Yes, but remmeber router will change destination MAC address at each hop. Only the last hop router will change destination MAC address in IP broadcast packet to ffff.ffff.ffff