setting up new workstation error

can someone help asap!
I have 2 client PCs set up fine by someone else no longer with us.Am trying
to add a new client but am doing something wrong.

I have set up new user and new computer on server (running windows small
business server) under admin tools,active directory.

On workstation i need to put in 2 DNS addresses.The first is the server
address.What is the second I need? I must have put in wrong numbers because
the server won't show the new PC's DNS name (which it does show on the
previously setup workstation).

Network cable is fine...I'm assuming it must be something to do with DNS
address on the client cos i can't think of anything else.

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Neil