Hi,
With a few exceptions, most intranets (or internal lans) will be using one of the rfc 1918 'private' address ranges :
10.0.0.0 - 10.255.255.255 (10/8 prefix)
172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
192.168.0.0 - 192.168.255.255 (192.168/16 prefix)
If you are using one of these ranges (or to be more pedantic any IP range that is not a 'real' allocated internet address range - some companies still incorrectly use other ranges for internal purposes) , then you need to employ some form of nat (network address translation) that will forward packets to/from a gateway machine and transparently swap the IP address from the internal one to the IP address of the gateway's internet interface.
You can do this with iptables quite happily using destination nat. You'd have an iptables rule something like this :
echo 1 > /proc/sys/net/ipv4/ip_forward
/sbin/iptables -t nat -A PREROUTING -p tcp --dport 80 -i eth1 -j DNAT --to 192.168.1.2
If you had that rule active on the mandrake box it would forward all tcp port 80 traffic transparently to the internal computer at IP address 192.168.1.2 (-i is the incoming , i.e. internet, interface). What you would also have to do is associate your site (via DNS or DDNS) with the public IP address of the linux box - you cannot point it at one the above address ranges because they have no meaning on the public internet.
Or is it something else you are looking for - its not 100% clear from your post.
Hope this helps