I was guessing.  Someone else explained how else it could be done.

"David A. Sinck" wrote:
> 
> \_ SMTP quoth Digital Wokan on 10/26/2001 09:41 as having spake thusly:
> \_
> \_ They can check to see if your system has a current lease on the DHCP
> \_ server.  If not, they can see if it's up with a ping.  If so, they know
> \_ you've got a static address set.
> 
> That's nice in theory, but my firewall crisps the ping packets too.
> :-)
> 
> David