Difference between NAT and NAPT?

[Marko Vojinovic]

On Tuesday 28 April 2009 07:51, Nifty Fedora Mitch wrote:
> On Sun, Apr 26, 2009 at 05:51:52PM +0200, Marko Vojinovic wrote:
> > In short, what is the difference? Are there any (dis)advantages of
> > using one over the other?
>
> Put your subject line in a search engine like Google.
>
>   http://www.dcs.gla.ac.uk/~lewis/networkpages/m05s09NAT.htm
>
> For most "mortals" NAT is just fine.
> NAPT may be needed on a large private network but
> the hardware/ software has to work harder and thus
> may cost more.

Well, I was already beginning to worry if my post made it to the list at 
all. :-)

Of course, I did do some research on the subject, but all that I found was 
described in a very general way, and consequently vague. So I was hoping to 
start a conversation with someone knowledgeable, in order to get more 
concrete answers.

My setup consists of three to five computers and a small wireless router, with 
an adsl uplink utilizing a dynamic public IP address (just a single one, the 
m=1 case in the article you quoted). What I would like to understand better 
is the following:

* Why does my ISP's router manual insists on using NAPT over NAT? The ISP tech 
support admitted to not understand why and have no explanation, but 
nevertheless they suggested that I set up the router as the manual says. Is 
there a general well-known reason for insisting on such a setup?

* Is there a performance penalty in using NAPT over NAT? Packets have to be 
altered and reassembled in both cases, so should I really expect any notable 
time difference here?

* Given my setup from above, is there a serious need to use NAPT over NAT? If 
yes, why? If not, why not? (note: I consider muself just a simple mortal with 
a small home network, nothing too fancy)

* I understood that NAT is about mangling the source IP address of the packet 
so one could push more local IP addresses through less public ones. I fail to 
understand the further gain of mangling tcp/udp port numbers? Can you provide 
an example situation where NAPT works and NAT doesn't, so I can visualize the 
difference in packet travel?

* Is it probable that the NAT setups I have created in the past (typically on 
Linux machines playing as routers, using mostly firestarter built-in NAT 
support) were actually NAPT setups, while I wasn't explicitly aware of the 
difference? IOW, is it maybe usual to say/write NAT in software manuals while 
actually meaning NAPT instead?

I tried to do my homework here, but these questions somehow just weren't quite 
answered in any NAT vs NAPT articles I could find on the net. I would 
appreciate any hints in understanding all this a bit better.

Thanks, :-)
Marko