How to set up a computer with proper domain name????

The exact details depend on the Linux distribution you're using. Maybe you can tell us more...

Is it if my linux distribution is red hat then what about it
thank you

In red hat / fedora the network configuration file is:
/etc/sysconfig/network
Set HOSTNAME= to your full domain name (like: "penguin.example.com") and restart the network.

I am using linux i.e red hat....ubuntu 10.04..
when i run the command /etc/sysconfig/network, it says that there is no such file or directory...also there is no such syconfig file or directory...

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