[Gllug] ADSL2 router help / recomendation

[Chris Bell]

On Wed 24 Jan, Mike Brodbelt wrote:
> 
> t.clarke wrote:
> > My understanding is that the Router DOES require an IP address on its
> > 'external' interface.
> 
> If it's a *router*, its a layer 3 device, and requires an IP address. It
> will in fact have an address for each routed interface it presents,
> though of course the internal interface is likely to use an RFC 1918
> address in a NAT configuration.
> 
> The external interface must be reachable by traffic coming to you. It's
> theoretically possible for a further layer of NAT to happen at your ISP,
> but of course any ISP that implements this sort of thing does so at the
> cost of degrading your IP connectivity, as many protocols you might want
> to use require a publicly accessible IP address (or specialist
> handling). There will always be stuff that would need special handling
> that won't get it, so any ISP that does not provide you with a routable
> address on your external interface is one to avoid, in my book.
> 
> > Our ISP actually provides two IP addresses for the line  (which sort of
> > alias to each other it seems).
> > Offhand I think the router uses one of those addresses on its external
> > interface and the Linux box uses the other address on its external interface
> > (to the router).
> > 
> > Maybe some routers work differently ?
> 
> No, they don't. They do if you use a mix of layer 2 and layer 3
> functionality, but for a plain router, you get one address (or more) per
> interface.
> 
> Mike

   I think the arrangement is that if you have a single public IP address
that connects via the BT version of ADSL, there is just one single route
from your local exchange and it must find that IP address, but it does not
need to be the modem or a connection through any kind of buffer that carries
it, as long as the modem does have some valid IP address, perhaps one in the
group 169.254.0.0/16. The box I have appears to be sufficiently configurable
to be able to isolate an interface and/or specify exactly what each is
allowed to connect and carry, similar to Linux bridge tables and IP tables.
I have used Debian Bridge Utils to do something similar. The spec does
include Bridge mode and Router mode, and it is sufficiently complex to make
it likely to run on some kind of embedded Linux.

-- 
Chris Bell

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