There are several motivations for building VPN’s

There are several motivations for building VPN's, but a common thread in each is that they all share the requirement to "virtualize" some

portion of an organization's communications – in other words, make some portion (or perhaps all) of the communications essentially

"invisible" to external observers, while taking advantage of the efficiencies of a common communications infrastructure.

The base motivation for VPN's lies in the economics of communications.  Communications systems today typically exhibit the

characteristic of a high fixed-cost component, and smaller variable cost components which vary with the transport capacity, or bandwidth,

of the system.  Within this economic environment, it is generally financially  attractive to bundle a number of discrete communications

services onto a common high capacity communications platform, allowing the high fixed-cost components associated with the platform to

be amortized over a larger number of clients.  Accordingly, a collection of virtual networks implemented on a single common physical

communications plant is cheaper to operate than the equivalent collection of smaller physically discrete communications plants, each

servicing a single network client.

So, if aggregation of communications requirements leads to a more cost-effective communications infrastructure, why not pool all these

services into a single public communications system?  Why is there still the requirement to  undertake some form of partitioning within

this common system that results in these "virtual private" networks?