is IFD required for federating another domain - when i have VPN between 2 domains no publishing at all

a very important point needs to be kept in mind, internet connection is not required in my scenario,  i will have vpn connection between domain 1 and domain 2, so nothing needs to be published from the CRM end, both data centers will be able to communicate over the VPN, in this case can i have the federation only to allow users in domain 1 access the CRM in domain 2, no IFD?

the link below for example assumes that internet is required for the communication between the 2 environments which is not my case

community.dynamics.com/.../ifd-claims-adfs-what-now

this is another thread with Karsten, but i missed mentioning that internet is not required in my case

https://community.dynamics.com/crm/f/117/t/210274