ce of a firewall. Many companies have links to other companies (parent companies, sister companies, partner companies, etc.). In each case, even if the companies are related, separating the main company from the others with a firewall is an excellent practice to adopt. Alternate Designs Figure 27-4 shows a simplified layout where Your Company's Network is connected to three other external entities. Firewall A is protecting Your Company from the In- ternet, Firewall B is protecting Your Company from the parent company, and Firewall C is protecting Your Company from the sister company. Internet Outside Inside Inside Outside To parent company Your Company's Network To sister company Inside Outside Figure 27-4. Multiple firewall example Each firewall has an inside and an outside interface. While each of the firewalls' inside interfaces are connected to the same network, the outside interfaces are all connected to different networks. Firewalls are also often used in multitiered architectures like those found in ecommerce websites. A common practice is to have firewalls not only at the point where the website connects to the Internet, but between the layers as well. Figure 27-5 shows such a network. In a layered design like this, one firewall's inside network is the next firewall's outside network. There are four firewalls connected to the balancing layer. The top two, a failover pair, connect the balancing layer to the Internet layer. To these firewalls, the balancing layer is the inside network. The bottom two firewalls (another failover pair) connect the balancing layer to the web layer. To these firewalls, the balancing layer is the outside network. Firewalls are another building block in your arsenal of networking devices. While there are some common design rules that should be followed, such as the ones I've outlined here, the needs of your business will ultimately dictate how you deploy your firewalls. Chapter 27: Basic Firewall Theory Internet Internet layer Outside Inside Trunk Balancing layer -Trunk Outside T