same way access from LAN and WAN

[Tieske]

I've got the following issue;
My homenetwork is behind a router which is connected to the providers modem,
my home-lan is in the range 192.168.50.xxx. I registered a domain name
linked it to my external IP-address (fixed) and configured my router to
forward the ports to my server, so I have it working succesfully with my
server; ftp.mydomain.com works perfectly when connecting from the internet.

But... when I get home and fire up my laptop (within my home network, so
from the same subnet), the only way to connect to the server is via its IP
address 192.168.50.2 connecting through the domainname no longer works. When
trying to reach the webserver by domainname for example, I end up on the
configuration page of the ADSL modem.

I tried using hosts files to fix this, so switching hosts files depending on
where I am, but for my room mates (anti-techy people) this is not an option. 
So I decided to install a DNS server on my homeserver (freeware called 
Treewalk). Many people warned me about the complexity, yet I want to give it 
a try.

After installing everything worked like before, so I assume the DNS server 
is working properly. But now I have to make some modifications to the DNS 
configuration so it will route any request for ftp.mydomain.com to the local 
IP address 192.168.50.2, without it querying any remote DNS which will 
probably come up with the external IP address.

Questions;
1) How do I register my domain with my DNS server? I read a lot of stuff on 
the record types etc., but where and how should I put this info?
2) I assume I should prevent my DNS server to pass these settings on to any 
remote DNS server?

Probably these are rather dumb questions for a DNS die-hard, even though I 
find them rather challenging, so any help would be greatly appreciated!