Advantages of VPN
VPNs promise two main advantages over competing approaches -- cost savings, and scalability (that is really just a different form of cost savings).
The Low Cost of a VPN
One way a VPN lowers costs is by eliminating the need for expensive long-distance leased lines. With VPNs, an organization needs only a relatively short dedicated connection to the service provider. This connection could be a local leased line (much less expensive than a long-distance one), or it could be a local broadband connection such as
Another way VPNs reduce costs is by lessening the need for long-distance telephone charges for remote access. Recall that to provide remote access service, VPN clients need only call into the nearest service provider's access point. In some cases this may require a long distance call, but in many cases a local call will suffice.
A third, more subtle way that VPNs may lower costs is through offloading of the support burden. With VPNs, the service provider rather than the organization must support dial-up access for example. Service providers can in theory charge much less for their support than it costs a company internally because the public provider's cost is shared amongst potentially thousands of customers.
Scalability and VPNs
The cost to an organization of traditional leased lines may be reasonable at first but can increase exponentially as the organization grows. A company with two branch offices, for example, can deploy just one dedicated line to connect the two locations. If a third branch office needs to come online, just two additional lines will be required to directly connect that location to the other two.
However, as an organization grows and more companies must be added to the network, the number of leased lines required increases dramatically. Four branch offices require six lines for full connectivity, five offices require ten lines, and so on. Mathematicans call this phenomenon a combinatorial explosion, and in a traditional WAN this explosion limits the flexibility for growth. VPNs that utilize the Internet avoid this problem by simply tapping into the geographically-distributed access already available.
Disadvantages of VPNs
With the hype that has surrounded VPNs historically, the potential pitfalls or "weak spots" in the VPN model can be easy to forget. These four concerns with VPN solutions are often raised.
1. VPNs require an in-depth understanding of public network security issues and proper deployment of precautions.
2. The availability and performance of an organization's wide-area VPN (over the Internet in particular) depends on factors largely outside of their control.
3. VPN technologies from different vendors may not work well together due to immature standards.
4. VPNs need to accomodate protocols other than IP and existing ("legacy") internal network technology.
Generally speaking, these four factors comprise the "hidden costs" of a VPN solution. Whereas VPN advocates tout cost savings as the primary advantage of this technology, detractors cite hidden costs as the primary disadvantage of VPNs.
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