If you need to setup an isolated network with VMs that still need access to a production network or the internet, you need to use a VM that is capable of routing traffic. Any number of possible solutions will work for this, a LINUX based firewall package like IPCOP or Smothwall, a Unix VM with routing configured or even a Windows VM with ICS or ISA server installed. To my mind the best solution out there at this time is Vyatta (http://www.vyatta.org/). Vyatta is an Open source package which is attempting to compete with Cisco in the routing product space. I will cover how to setup Vyatta to act as a network bridge between a vswitch with no VMNIC uplink and a vswitch with an uplink to the production network.