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Q & A Networking · September 11, 2026

What is the difference between a public and private IP?

Public IPs are routable on the internet — any host on the internet can reach them (subject to firewalls). Private IPs are reserved for internal use only and never routed on the internet. RFC 1918 defines the private ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16. Your home router probably assigns you a private IP like 192.168.1.42. Your router has a public IP on its WAN interface. NAT translates between the two. Behind CGNAT (common on cellular), even your router’s WAN IP might be private. Read the CIDR subnet math article for the address space details. Find your public IP with the What Is My IP tool.

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CIDR Subnet Math Explained
CIDR notation is how modern networks are described: an IP address plus a slash and a number counting the network-prefix bits. Everything left of the boundary is the network; everything right is the host. The math is simpler than the notation makes it look.
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