DNS problem

[ZaiD Dashti]

hello

i have a problem with my DNS server.

first, i have a registered domain name and i want to change its NS server to 
my server to host it in my home (just for learning about DNS)

second (the problem), when i use nslookup from another computers in my local 
network, the dns
server works fine, but when external connection (internet connection, let 
say my friend connection) tryies to use my dns server, i got error message 
"DNS request timed out", why ? and how to solve it ?

NOTE: i have an ADSL account, which mean i'm using a router to connect to 
the internet, and i'm redirection all ports from the external connections to 
my machine ip, so that i'm able to open a server or to connect to my machine 
from anywhere.

thanks