Hosted cloud services like Amazon's EC2 continue to garner most of the headlines, but the prospect of internal cloud architectures -- those located behind the firewall -- is gaining credence as the next major phase of enterprise evolution.

Part of this trend is due to the inevitable backlash that new technologies experience when talk of their inevitability reaches critical mass. And while it's true that external cloud computing offers a number of advantages over traditional data centers (namely, that you really don't need one anymore), the fact that few established enterprises are willing to chuck their network investments to pursue a cloud strategy indicates that the hosted offerings will have limited appeal for some time.

In fact, as Cassatt Corp.'s Ken Oestreich points out in this blog, if you look back at the history of many enterprise services like Google Search and Software as a Service (SaaS), the trend for hosted applications has been to migrate into the enterprise, not the other way around.

How, then, do you create a cloud on legacy network equipment? Well, beefing up I/O capabilities is a good start. 10 Gbps Ethernet or Infiniband would be your best bets right now, preferably with a virtual I/O platform that allows you to easily scale network resources up and down to meet demand.

Your storage is probably the best candidate to begin your experiments with cloud architectures. ParaScale's Mike Maxey says there are two main options for cloud storage. Tightly Coupled Symmetric (TCS) architectures use multiple nodes with distributed lock management and cache coherency to overcome I/O limitations in single-file environments. Loosely Coupled Asymmetric (LCA) architectures rely instead on an out-of-path central control server to afford faster and better scalability.

The vendor community is already warming up to the idea of internal clouds. Start-up 3Tera is repurposing its AppLogic system, which traditionally went to host services like Nirvanix, for corporate users to provide grid services on commodity server platforms.

Heavyweights like IBM are also keyed in to the possibilities of internal clouds. The company recently launched a pair of new cloud centers that it will host out to others, but it also is busy touting the New Enterprise Data Center (NEDC) program that leverages the company's virtualization and network management tools into a kind of "do-it-yourself" cloud architecture.

There are those that say the internal cloud is now the inevitable evolutionary fate of the modern data center, which has us wondering how long it will be before the backlash against that notion wells up. But as the twin demands of increased performance and decreased energy consumption continue to grow, it's a good bet that both internal and external clouds will make their presences known at a data center near you before too long.