DNS for my clients sub domain

HI

I am trying to configure my DNS to manage my clients subdomain so that it routes users to the correct site.

Let me explain my question by way of an example:

I host abc.com for my client.
My client's main domain is 123.com
They want abc.123.com to point to the site i host for them.
I gave them my servers ip 123.456.789.101

But where/how do i configure my server to respond to abc.123.com correctly?

Do i use the Parked Domain feature or do i Add a DNS Zone file duplicating the one i have from abc.com but changing it to abc.123.com?

Thanks for you help.

atilla