[lug] cable modem network topology

D. Stimits stimits at idcomm.com
Fri Jul 19 10:26:19 MDT 2002

Within the last two weeks, my telephone line quality went permanently 
downhill. Not only is it between 25% and 33% slower, latency seems to 
have doubled. The phone company is not interested until it drops below 
14.4kbps. Within roughly the last week, cable modems became available 
for a good price (especially compared to DSL).

What I want to do is use an old P166 as the firewall/router/gateway, but 
it is complicated by the need for 3 IP addresses, all of which are 
dhcp/non-static. Each dynamic address beyond the first costs $5 each, 
but that is fine for 3 computers that might run at the same time. The 
gateway/router/firewall does not need a routable IP as far as I am 
concerned. What I wanted was something like this:

cable modem
     | (eth0)
   P166 firewall/gate/router
     | (eth1)
   8 port switch
     |- Machine 1
     |- Machine 2
     |- Machine 3

But how to actually do this is a mystery, it seems as though the P166 
would need eth0 to respond to multiple dhcp IP's, and then transparently 
forward them to whichever machine booted up, while still allowing rules 
to stop things like port 137-139 from passing through. I have never set 
up a DHCP system, which seems easy if only one machine touches the cable 
modem, but becomes problematic if the P166 must simply pass through DHCP 
  packets, then do the right firewalling for each machine. Can this be done?

D. Stimits, stimits @ idcomm.com

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