This should be remarkably simple but I seem to have myself turned around
on it.
I have a Mac 9600 with 2 ethernet interfaces (mc0 & ep0).

mc0 is configured a public network at aaa.bbb.ccc.42.
ep0 is configured for the internal network at 192.168.1.1

gateway is aaa.bbb.ccc.41

dhcpd is enabled and dhcp client machines can connect to the server at
either address.
dhcpd.conf is as follows;

  allow unknown-clients;
  ddns-update-style ad-hoc;

  option subnet-mask 255.255.255.0;
  default-lease-time 3599;
  max-lease-time 604800;

  # Set parameters for the 192.168.1.0/24 subnet.
  subnet 192.168.1.0 netmask 255.255.255.0 {
        range 192.168.1.2 192.168.1.254;
  }

ipnat is configured as;

  map ep0 192.168.1.0/24 -> 0/32

Although I can ping aaa.bbb.ccc.42 I can't ping aaa.bbb.ccc.41.  i.e. my
packets don't really seem to be forwarding.  I've tried other mappings
with no luck.
What am I doing wrong?