Private vs public IP addresses
A private IP address only means something inside your own network; a public IP address is unique and routable across the whole internet. Almost every device you own has both at once, one private and one shared public address, connected by NAT.
What makes an address private
IANA set aside three blocks of IPv4 address space, defined in RFC 1918, that routers on the public internet are configured to never forward: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Any device can use an address from these ranges on its own local network without asking anyone's permission, because the same address can be reused on millions of other private networks at the same time without conflict.
That reuse is exactly why the ranges exist. When RFC 1918 was published in 1996, it was already clear the internet did not have enough public IPv4 addresses for every device on every home and office network, so private ranges let organisations number their internal networks freely while only spending public addresses at the edge.
The three private ranges
10.0.0.0/8 is the largest, 16,777,216 addresses, and is the usual choice for large corporate and cloud networks that need room for many subnets. 172.16.0.0/12 holds 1,048,576 addresses and sits in the middle, less common but still seen in mid-sized deployments. 192.168.0.0/16 holds 65,536 addresses and is the default on almost every home router, usually carved down further to a single 192.168.1.0/24 or 192.168.0.0/24.
Two related ranges are private in spirit but reserved for a specific purpose rather than general use: 127.0.0.0/8, the loopback range where 127.0.0.1 always means this device, and 169.254.0.0/16, the link-local range a device assigns itself automatically when it cannot reach a DHCP server.
How a private address reaches the public internet
A private address cannot be routed on the internet: no public router carries a route for 10.0.0.0/8, so a packet from a private address would have nowhere to go. Network Address Translation (NAT), running on your router or firewall, solves this by rewriting the source address of outgoing traffic to the router's own public IP address before it leaves the local network, then translating replies back to the right private address on the way in.
That is why a laptop can show a private address like 192.168.1.42 while an external site reports a completely different, public address: the private address is what the laptop knows about itself, and the public address is what the rest of the internet sees.
How to tell if an address is private
Check whether it falls inside 10.0.0.0/8, 172.16.0.0/12, or 192.168.0.0/16. The subnet calculator on this site flags an address as private automatically when you enter it, using exactly these three ranges plus loopback and link-local.
Everything outside those ranges, aside from a handful of other reserved blocks such as documentation ranges and multicast, is public: assigned to an organisation and, if actually routed, reachable from anywhere on the internet.
Frequently asked questions
What are the three private IP ranges?
10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16, defined in RFC 1918. Together they total roughly 17.9 million addresses set aside for use on private networks.
Can two devices on different networks share the same private IP?
Yes, and it happens constantly. Millions of home routers use 192.168.1.1 for their own gateway address; it causes no conflict because private addresses are never routed between networks directly.
Is 127.0.0.1 a private IP address?
It is reserved rather than private in the RFC 1918 sense. 127.0.0.0/8 is the loopback range, and 127.0.0.1 always refers back to the device itself, never to another host.
How do I know if my IP address is public or private?
The address your router shows for a device on your LAN, often starting 192.168 or 10, is private. The address an external site reports back to you is your public address, the one NAT translates to.