Redirect domain name mapped with external IP (modem) to a host in its local network

Redirect domain name mapped with external IP (modem) to a host in its
local network

I have a modem that is connected to a router. There are two computers
connected to this router. I want to set a domain for one of those
computers so that I can access the web deployment server running in that
computer from Internet. I used freedns.afraid.org to get a free subdomain.
But when I set it up, the domain is getting mapped to my external IP
address. So, the problem is that when I try to access the set domain, I am
only accessing the modem's configuration page i.e. EXTERNAL_IP:80
In my local network, my modem's IP address is 192.168.0.1 , my router's IP
address is 192.168.1.1 and my computers take IP addresses anywhere between
192.168.1.100 and 192.168.1.148
Also, my router is Linksys WRT54G and is only capable of default DDNS
configuration to dyndns and TZO. Since both have become paid services, I
opted for free DDNS and I'll use some DDNS update client to update the
dynamic IP.
I don't know how to search specifically for this problem in google. Also,
I came across port forwarding which was used under similar topics. I am
new to this, so can anyone suggest me how to redirect my domain to the
particular host or is there any reference page I can learn the procedure
from?