t is one of the best ways to accomplish the required protection. The standard design for an ecommerce website is composed of three tiers. The first tier contains the web servers, which are reachable from the Internet. This is called the Internet layer. The second tier contains the application servers, which cannot usually be reached from the Internet. These servers can talk to the web servers above them and the database servers beneath them. The lowest layer is the database layer, which can be accessed only from the application layer. There are many ways to design an ecommerce website. The network design will be determined, in large part, by the software in use and the developer's methods. I've worked on ecommerce websites that had only two layers and others with four. Sometimes, developers will insist on a single layer. If the application simply does not support multiple layers, forcing the issue will only waste time. Still, when dealing with servers that store credit card data, make sure your design adheres to Payment Card Industry (PCI) standards for security. The Internet layer is the least secure layer, because it is accessible from the Internet. Even though this layer is protected by a firewall, the general population has access to services residing in this layer. The most secure layer is the database layer. The only way to access the database layer is through the application servers. These servers will have Network Designs special applications residing on them that can process and possibly cache database information for the web servers. The three-tier ecommerce architecture is shown in Figure 35-11. The Internet layer contains the most servers. The number of servers generally decreases in the lower layers, though this is not a universal truth. For example, the database layer may be composed of an Oracle cluster, which could use many smaller servers. Internet Less secure Internet layer Web servers Application layer Application servers Database layer Database servers Most secure Figure 35-11. Typical three-t