Computer Firewall Question:

Answer #1
A network that appears to be a single protected network behind firewalls, which actually encompasses encrypted virtual links over untrusted networks.

Answer #2
Virtual Network Perimeter can be divided in two words: Virtual Network which means that we have logically, different separate physical network and perimeter which means that they are on the edge of our network( they have a way to the outside world) and should be protected by Firewall and IDS.
In the reality, we have one physical structure which is shared by different logically separated network. So, it makes sense that we share the services such as Internet, DHCP , server farm and Management by these VNs.
Typically, the perimeter is deployed at a common physical location for most VNs. Hence, this location is known as the central services site, and the security devices here deployed can be shared by many VNs.