I am trying to use a IIS reverse proxy running on IIS in a new Azure VM against an old classic VM. I have the server proxying onto the classic VM but this is currently over the public IP address.
Is it possible to get a new Azure VM, running in a resource group (on one subnet) talking to a classic VM running in a Virtual network (classic) section on a different subnet?
Yes it is possible. This is the most up to date instructions that I could find Bridging the Gap between Azure Classic and ARM. Beware: there are other sets of instructions that have out of date Powershell commands.
Using the bridge would allow your VMs on both sides of Azure to be on the same domain and therefore communicate to each other.
I successfully set up this bridge to use Azure AD Domain Services (only in classic) with VMs running in Azure Resource Manager (ARM).