Jeff Dunlop wrote in a message to All:
 JD> How does a workstation configured to use DHCP actually find
 JD> a DHCP server without an IP address and netmask to find it?
 JD> Is there a special broadcast/response protocol?
DHCP assumes the existence of some sort of hardware address at the link layer 
which uniquely identifies each node.  In the case of Ethernet, this is the 
48-bit MAC address burned into each card.
Usually, a client which wants an address assignment sends a DHCPDISCOVER 
broadcast claiming to be from 0.0.0.0 to 255.255.255.255.  Any local server 
which hears this directly will glean the hardware address from that 
broadcast, and will unicast a DHCPOFFER reply containing the proposed IP 
address.  Some clients cannot handle the unicast reply, so they set a flag 
asking the server to broadcast the reply instead.
 JD> What if the DHCP server isn't on the same physical segment, and 
 JD> the negotiation must take place through a router? 
If there is no local DHCP server, some machine on the local segment -- almost 
always the router -- is responsible for encapsulating the request and passing 
it on to the appropriate DHCP server as normal traffic.  The DHCP server then 
sends a reply wrapped similarly, and the client has the illusion that the 
DHCP server is on the local segment.
See RFC1541 if you want the low-level details.  DHCP is a superset of BOOTP, 
so reading RFC951 may also be useful.
 
-- Mike
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