Let's look at an IPSec VPN tunnel implementation example:
The cloud icon represents a public unsecured network, say the Internet. Let's say there are two remote office sites, Site A and Site B. This could be two branch offices of a company situated in different cities or even different countries. Both of these remote sites have their own independent ISP connections. They also have a local pfSense firewall implemented in remote locations. Similarly, they also have an internal LAN, consisting of various internal network devices. Site A has the internal IP address range of 192.168.1.0/24. Site B has the internal IP address range of 192.168.2.0/24. In this scenario, these two sites are protecting their local internet networks with an installation of pfSense. But what if these two sites need to connect to each other? Or all the branch offices...
