Automatically connect external hard drive over network?

I recently added a MBP to my Apple family. I really like it. I have a wireless network setup in my house. I have an external hard drive hooked up to my iMac and am sharing via the network. I am able to "see" and connect to the external hard drive with my MBP without a problem. In preparation for time machine, how can I setup my MBP to connect to the external hard drive automatically when it connects to my network?

thanks.

 

I was told by the mac geniuses that I would need an airport extreme in order to automatically mount to both the iMac and MBP