not always the outside interface 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 best practice to adopt. Figure 25-4 shows a simplified layout where Your Company's Network is connected to three other external entities. Firewall A is protecting Your Company from the Internet, 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 To parent company Outside Your Company's Network To sister company Inside Outside Figure 25-4. Multiple firewall example Alternate Designs Each of the firewalls has an inside and an outside interface. While each of the fire- walls' 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 e- commerce web sites. A common practice is to have firewalls not only at the point where the web site connects to the Internet, but between the layers as well. Figure 25-5 shows such a network. Internet Internet layer Outside Inside Trunk Balancing layer "Trunk Outside Trunk Inside Web layer Outside Trunk Inside Application layer Outside Trunk Inside Database layer Figure 25-5. E-commerce web site In a layered design like this, one firewall's inside network is the next firewall's out- side 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