How to NAT inside a LAN over a single Interface

[Dietmar Hofer <didi156@gmx.net>]

I haven't found anythink clearing my problem doing some research in this 
list, nevertheless I'm sorry if you find my question annoying 'cos I'm 
quite new to this issue.

I'm in a class B LAN and would make a Machine work as Gateway for 
another, both in the same network. This because the Internet Gateway 
accepts only requests of registered Interfaces (MAC-based).
The Machine which I want to let do this has only one eth-Interface. what 
in theory should be enough.
I set up NAT with "iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE" 
and changed the route on the source machine to use the other as gateway.
When pinging from the source machine, "/var/log/syslog" on the gateway 
shows me this requests:

Dec 18 22:42:44 hogwart kernel: IN=eth0 OUT=eth0 SRC=192.168.2.201 
DST=192.168.2.150 LEN=8
4 TOS=0x00 PREC=0x00 TTL=63 ID=0 DF PROTO=ICMP TYPE=8 CODE=0 ID=7294 SEQ=1

But I don't get an answer on the source machine, (while naturally i can 
ping the given IP from the gateway itself).
In a HOWTO I found the hint that doing NAT with only 1 Interface for 
input and output may not work with this config 'cos since kernel 2.4 
some sort of ICMP redirections doesn't work or so...

What I want to know is just what I've to do to use this machine as 
gateway with only one interface.
Hope you can help