Traffic between nodes

Hi, I'm new to Linode service, but I used a lot of others VPS services, for some time now.

I was wondering… The traffic between "nodes" is charged, entering on traffic bandwidth limit or it is free inside the same data center?

I was planning to setting up a VPN between all my nodes (and some real machines) to do some distributed job, but that could imply in some huge data transfer between the nodes.

Thanks!