Accessing guests on virtual network when connected to host via PPTP

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http://serverfault.com – I'm setting up a development machine which runs Ubuntu 12.04 and KVM for virtualization. I have a guest running Ubuntu 12.04 which can be accessed from the host via its IP address which is assigned by libvirt. The guest can also access the internet, no problem there. However, now I want to setup PPTP so I can connect to the host (from my workstation running Windows 7) and directly access guests without relying on SSH port forwarding. I can connect from my W7-machine to the host (PPTP), but I cannot access any virtual machines (which are accessable from the host directly). Relevant configura (HowTos)