Scanning hosts behind a NAT

Hi list ;),
I'm a student and I'm trying to learn how nmap does its job.
Today, for example, I tried to scan my home network ( ;) )... In fact,
I've 2 computers behind a router (which does wireless AP, router &
firewall: linksys wrt54g). Then, I tried to scan from "outside" the
network (aka: from a friend on the internet).
On the router (LAN ip: 192.168.1.1) , I've the port 6356 (Gnutella)
which is forwarded to 192.168.1.2 (my first pc).

When I tried to scan from outside, I obviously obtain:


Starting Nmap 4.01 ( http://www.insecure.org/nmap/ ) at 2006-04-22 18:26 CEST
Warning:  OS detection will be MUCH less reliable because we did not
find at least 1 open and 1 closed TCP port
Interesting ports on 80.13.xx.yy:
(The 1671 ports scanned but not shown below are in state: filtered)
PORT     STATE  SERVICE  VERSION
6346/tcp closed gnutella
Too many fingerprints match this host to give specific OS details

In fact, it was what I was expecting for. My question is how to scan
the hosts behind the router (NAT) ? Is it possible ?

Thanks

N.

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