whitelist IP address

Hi,

I have a LAMP stack and have a subdomain/ virtual server set up as part of the website.  I would like to setup a whitelist for the subdomain and only allow one IP address to see the files on the subdomain.   We have a situation where this one allowable ip will be pulling down files and I don't want to make those files public.  

Can someone point me to the right direction?

Thanks  rg