How does my computer work with an empty arp table?

[Chuck Swiger]

On Dec 12, 2006, at 10:08 AM, Javier Henderson wrote:
>> The ARP table only contains information about machines on the  
>> directly connected collision domain(s).
>
> Are you sure it's not the same broadcast domain?

Yes.  The term "collision domain" predates the wide deployment of  
switches, and switches have to treat ARPs in a special fashion:

> A computer on port A on a switch would be on a different collision  
> domain than a computer on port B on the same switch, yet as long as  
> they're on the same VLAN (ie, broadcast domain), both would have  
> each other in their resepctive ARP tables if they were exchanging  
> Ethernet traffic.

...in particular, ARPOP_REQUEST traffic will be propagated to every  
port on the switch which is configured to be a part of that VLAN, or,  
quite possibly, other ports including "trunk ports" or sometimes even  
ports configured on other VLANs. [1]

Many switches will do this for all ethernet packets with an  
ether_dhost (ie, destination MAC) of all-ones.

-- 
-Chuck

[1]: And yes, Virginia, this has negatory implications if your  
security relies on VLANs to actually be completely hidden from each  
other.