I've got a project coming up that requires me to protect hosts from each other 
within a network. Specifically, we've a class C subnet, and some addresses are 
assigned to customers (only a handful) we resell bandwidth to. At present they 
are just plugged into our frontend network - not as bad as it sounds as we 
manage the customer routers involved. However, I want to improve that, so that 
"misconfiguration" of any customer device cannot take out our network - not 
that I'd ever fail to notice an old router where the gateway address box is 
first in list (ie gateway is where device IP normally is, and vice-versa) and 
so configure it with a duplicate IP address for our gateway (oops).

So my plan is to knock up a small box, with a VLAN capable switch, so that each 
customer has their own network segment. What's the best way to configure this ? 
As I see it, there are two approaches :

1) Bridge.
Configure all the customer VLANs and our frontend network on a bridge, and 
filter the traffic to only allow the specific IP (or IPs) to owrk in each VLAN.

2) Proxy ARP
http://shorewall.net/ProxyARP.htm
which seems like it'll do the job.

Just for good measure, ideally I'd like to get DHCP working so each customer 
can "just plug in" and we don't need to manually configure their router for 
them. I'm well flummoxed on how to make that work ! but that's a different 
mailing list. Might need a DHCP instance per port.

So expanding on the example in the Proxy ARP page, I want it so that the device 
at130.252.100.18 can only use that address. If it gets configured 
with130.252.100.19 or worse,130.252.100.17, it won't "take out" the network but 
will just "not work". Does the proxy ARP setup provide that level of protection 
? I don't need any other filtering - they are outside of our main firewall etc 
(so the policy will be allow any->any).

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