The Internet is 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 practice to adopt.
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 Internet, Firewall B is protecting Your Company from the parent company, and Firewall C is protecting Your Company from the sister company.
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.
Figure 27-5. Ecommerce website
In a layered design like this, one firewall’s inside network is the next firewall’s outside network. There are four firewalls connected ...