Thanks for asking your question on the forum!
After doing some research, it looks like Chromecast uses a multicast discovery packet which discovers clients who want to cast. This multicast packet has a TTL of 1 so it cannot be routed across subnets.
However, I believe Google is looking to add a feature soon that will take away the requirement of being on the same wireless network/subnet.
Please do not hesitate to let us know if you have any questions.
Thanks Noor !
I appreciate the reply - I will have to dig into this a little more - I hope they make the change!