How do I tell my system its domain name ?

[John French]

I want to use a Gutsy system as a Samba server, and as my domain 
controller. I think I am getting smb.conf somewhere near what I need, 
but I can't seem to get the beast to learn its domain name.

I have put A50.tow.org.uk in /etc/hostname, and I have

192.168.0.240   A50 A50.tow.org.uk

in /etc/hosts (A50 has a fixed IP address). But whenever I run

$ hostname --fqdn

I get just A50.

How do I get the domain name accepted ?

John French