TUMBLEWEED Mapping external IP's to internal ones

I have got myself a bit lost - this is what I want to do ... ( IP's changes to protect the innocent )

I have a box with 2 network cards
the internal interface is 1.2.3.254
the external interface is 5.6.7.8

inside there are 1000 or so private IP addresses
outside I have 3 public IP addresses

at the moment it is set up as a router with the firewall on
internal machines can surf Ok and if I use 'test my IP' they all
show up as 5.6.7.8 which is what I expect and everything seems OK

Now I want to make my LDAP server visible to another user on the Internet
so I essentially need to map 5.6.7.9 to 1.2.3.4 so the Internet company think
they are talking to 5.6.7.9 when in actual fact they are talking to the LDAP
server inside

Idiots guide please :-)

Ta

mal