RFC 1918 (Address Allocation for Private Internets) explicitly reserves three blocks of IPv4 address space for private enterprise networks that do not require direct connectivity to the global Internet and are filtered by default on public Internet routers:
1. 24-bit block (Class A equivalent): 10.0.0.0/8 (spanning 10.0.0.0 to 10.255.255.255).
2. 20-bit block (Class B equivalent): 172.16.0.0/12 (spanning 172.16.0.0 to 172.31.255.255).
3. 16-bit block (Class C equivalent): 192.168.0.0/16 (spanning 192.168.0.0 to 192.168.255.255).
Option C (102.0.2.0/24) is a publicly routable range allocated to public registries. Option D (172.0.0.0/16) is a publicly routable space, as private Class B addresses start strictly at 172.16.0.0. In WatchGuard Fireware, RFC 1918 address ranges form the foundation for interface configuration on Trusted and Optional networks, default dynamic NAT rules, and private network definitions across Mobile and Branch Office VPN tunnels.
[Reference: WatchGuard Network Security Essentials — Network and Network Security Basics: RFC 1918 Private Addressing and Dynamic NAT Architecture., ――――――――――――――――――――――――――――――――――――――――――――――――――――――――――]
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