Pen tester Mark Wolfgang argues segmenting for security is a key piece of an overall defense-in-depth strategy. Here he explains why and how to accomplish it in your organization (registration required)
My job over the last thirteen years as a penetration tester has given me a unique understanding of security from an attacker's point of view. I have conducted hundreds of penetration tests on organizations ranging from Federal government nuclear weapons labs, to banks, city governments, and practically everything in between. I know what makes an attacker's job easy, and what makes it difficult or practically impossible. I am oftentimes surprised that in 2014, I can gain access to one server or workstation, and use it to traverse the entire network, unhindered at the network layer.