Private and public IP addresses: what they are and how NAT connects them

Your phone and laptop have private addresses. The address the internet sees is the public one on your router.

Public addresses

A public IP address is unique on the internet and can be reached from outside (unless a firewall blocks it). Internet providers receive blocks of public addresses and give one to your home router or office router.

Private addresses

RFC 1918 reserves three blocks of private addresses for use inside organisations and homes:

The same private addresses are used in millions of separate networks. They are not routed on the public internet: an address like 192.168.1.5 means different devices in different homes.

NAT: how private devices reach the internet

The router at the edge of your network does network address translation. When your laptop (192.168.1.5) opens a web page, the router rewrites the source address to its own public address, remembers the connection, and when the answer returns, it forwards it to the laptop. Many devices therefore share one public address. A side effect is that outside computers cannot start a connection to your laptop unless you set up forwarding.

Two other ranges to know

• 100.64.0.0/10 is the "shared address space" of RFC 6598 (listed in the IANA registry). Some providers use it between your router and their equipment ("carrier-grade NAT"). If your router's WAN address starts with 100.64 to 100.127, you do not have a public address of your own, which matters for hosting something at home.
• 169.254.0.0/16 is link-local: a device gives itself such an address when it cannot get one from a server. Seeing one usually means "the network did not give me an address".

Checking which one you have

Your device's address (from the network settings) tells you your private address. The address shown by "what is my IP" websites is your public address. If the two are very different (192.168.x.x against a public address), a NAT is in between. The IPv4 calculator on this site labels any address as private, public or special.

Security notes

NAT is not a firewall, although it behaves like one for incoming connections. Do not rely on a private address for secrecy: anything inside the network can reach other private addresses unless you separate them.

IPv6

IPv6 has so many addresses that NAT is usually not needed; devices have globally reachable addresses protected by a firewall. IPv6 also has private-like ranges (unique local addresses fc00::/7).

This is an independent guide. It is not connected with IANA, the IETF or any network equipment maker. Address and port data come from the IANA registries as fetched on 14 November 2026; RFC texts were read from rfc-editor.org. Check the originals for anything you rely on.