Creating Encrypted Time Machine Back Ups to a Network Hard Drive

Hello all, I can't seem to create an encrypted Time Machine back up on a network hard drive. Can this be done? And if not, why not? It seems that Macs have all the necessary functionality already available in Mac OS. Time Machine can create encrypted backups with multiple computers so what's the difference if it is to a network backup drive, honestly? Any suggestions?

My Set up: Three Macs, Airport Extreme Base Station, Seagate 3 TB HD (with separate partitions for each computer's backups). 2 of the Macs use ethernet to connect to the network and one uses wireless regularly. Basically, I'm trying to encrypt my family's backup drive and I don't know how or why I can't.

Thanks in advance for the help.

Appleguy514