A firewall prevents unauthorized access to the corporate network, and in the Security+ exam, we tend to use a back-to-back configuration, as shown here:

Figure 2: Back-to-back firewall configuration

You can see each of these firewalls is not letting traffic pass through them; this is because we need to open only the ports that we need. If Firewall 1 is traversed, then Firewall 2 will hopefully prevent access to the internal network known as the local area network. To enable applications to pass through the firewall, we must open the port number for each application. Each application has a different port number, if you think of someone ...

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