DSL: PPP versus Bridging was Re: Getting DSL configured using Linux

[James Lee Bell]

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As many of us appear to be, I'm checking into DSL, and I've seen
comments regarding running the Cisco 675's (model num correct?) in
either PPP or Bridging mode. I think I've got these definitions correct
in my head, but please correct me if I'm wrong.

PPP mode: Router gets the public IP address on it's external interface,
and it's internal interface (rj45) gets set to one of the privately
usable networks, i.e. 10.* The router can then do your port blocking,
can do NAT, and can even do DHCP serving if you want it to.

Bridging mode: Router acts as a bridge, passing any and all packets from
one interface to the other regardless of packet contents ("packets is
packets"), and doesn't exist as an entity in and of itself as far as IP
networking is concerned. So any assigned IP address goes to the external
NIC of the firewall (if present) or the first machine turned on if crazy
enough to plug router directly into a hub.

So it seems to me somewhat arbitrary and up to the ISP as to which mode
is used, except when thinking about having a firewall or not.  If you're
not planning to run your own firewall, then PPP sounds good because you
can do some basic firewalling using the router.  But if you're planning
to run your own firewall for whatever reason (granularity, logging, ...)
then you can work with either mode, i.e. if forced to run it in PPP mode
you run without any filters and probably without NAT at that level (just
for speed) and let your firewall do any filtering, forwarding, port
mapping, NAT, etc.  And if you must use the router in bridging mode,
there's no difference to your firewall other than it's external nic gets
the public IP address.
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