[X4U] What is NAT and what does it mean to me?

[David Ledger]

At 00:25 -0700 11/5/07, x4u-request at listserver.themacintoshguy.com wrote:
>From: Tim Collier <timjcollier at mac.com>
>Subject says it all.  I installed BellSouth DSL and connected the
>wired modem to my 802.11n Airport Extreme.  It kept giving me one
>error that said Double NAT.  I called BellSouth and they told me that
>NAT was not enabled on the modem and I didn't turn it on as an option
>on the Airport (as I really don't understand what it is) but after I
>applied the settings, it came up as an error.  It gave me the option
>to ignore it and I checked that, I then updated the Airport again and
>now everything is fine.  The light on the front is GREEN....that's
>GOOD.  But still, what is NAT and what does it mean to me?

If it's just a modem it won't do NAT. (Although with the people who 
name products knowing so little about technology these days, anything 
is possible :-) ). Anything that does NAT must have different IP 
ranges on each side and so qualify as a router or even a gateway. If 
that and the Airport are all the network kit you have then I can't 
see how you can have anything that could cause any error that could 
be described as 'Double NAT'. What item told you of the error?


>From: Robert Ameeti <robert at ameeti.net>
>Double NAT would be where the ISP's router was doing NAT and
>providing a 'network' of addresses and then one of the devices (your
>AirPort) hooked up to one of its ports was also doing NAT. It is not
>impossible to deal with but should be avoided when possible.

It's fairly straightforwards to deal with. You just have to think 
about what is going on. The main problem is actually connecting to 
the components while you're setting it up. I have worked at a site 
where we had to investigate an odd network problem. As part of the 
investigation someone noted that packets were getting NATed six 
times. It was a large complex network.


>From: Eugene <list-themacintoshguy at fsck.net>
>	client A (e.g. Mac) <-> NAT B (e.g. Airport) <-> NAT C (e.g. 
>modem) <-> Internet
>	client A <-> NAT B <-> Internet <-> NAT C <-> client D

Strictly, the Internet is irrelevant, but as Eugene points out, a NAT 
function is one to many and so can be either way round. The complex 
network I mention above will have had Internet connection on one of 
the arms, but it was not significant. Sometimes it is necessary to 
abandon the popular use scenario, its ideas and product 
'documentation', in order to fully understand and discover the source 
of a problem.

David


-- 
David Ledger - Freelance Unix Sysadmin in the UK.
Chair of HPUX SysAdmin SIG of hpUG technical user group (www.hpug.org.uk)
david.ledger at ivdcs.co.uk
www.ivdcs.co.uk