Addresses

Public vs Private IP

The address on this page is a public IP address: the one this connection presented to the internet. The address in your phone’s Wi-Fi settings is usually a private IP address, used only between the phone and the router. They are different numbers on purpose. This page is the difference, the ranges, and the two kinds of translation that sit between them.

Public means reachable, in theory

A public address is drawn from the space regional registries allocate to ISPs and other networks. Packets sent to it can cross the internet, as far as routing and firewalls allow. When you open a website, the site sees this address as the source. When someone wants to connect to you, they need a public address that lands on your machine, plus a path through every firewall in between. Most home routers drop unsolicited inbound traffic even when the WAN address is public. “Public” does not mean “my camera is already on the internet.”

Private ranges

These IPv4 blocks are reserved for networks that are not on the public internet:

• 10.0.0.0/8 — 10.0.0.0 through 10.255.255.255. Common on larger offices, some ISPs’ internal links, and some home routers.
• 172.16.0.0/12 — 172.16.0.0 through 172.31.255.255. Not the whole of 172.x. 172.15 and 172.32 are outside the private block.
• 192.168.0.0/16 — the range most home routers use. 192.168.0.1 and 192.168.1.1 are typical admin pages.
• 169.254.0.0/16 — link-local. A device that could not get a DHCP lease picks one of these. It only talks on the local link.

IPv6 unique local addresses sit under fc00::/7 (you will usually see fd). Link-local IPv6 addresses start with fe80. A fe80 address is real and useful on the LAN. It is not what a website sees.

NAT is the translator

Network address translation is how a household of private addresses shares one public address. The router rewrites the source of outbound packets and remembers the reply path. Every laptop can browse. An outsider who sends a packet to the public address hits the router, not a particular laptop, unless you forwarded a port or the laptop opened the flow. That is ordinary home NAT, and it is why the number in ipconfig and the number on this site’s homepage are different. Both are “your IP” in casual speech. Only one of them is public.

CGNAT is a second translator

100.64.0.0/10 (100.64.0.0 through 100.127.255.255) is reserved for carrier-grade NAT. The ISP runs another translation for many customers at once, because it does not have a public IPv4 per customer. Your router may think its WAN address is 100.64.x or even a 10.x address the ISP used internally. This site shows the address after that second translation. Matching numbers mean there is no second translation. Different numbers mean there is. The CGNAT check is that comparison, with notes for networks where it is common.

Sharing has a privacy side effect and a reachability cost. A website cannot tell two customers behind the same public address apart from the number alone. You also cannot invite a connection in. IPv6, where the line has it, is the path that was designed to avoid this squeeze. IPv4 vs IPv6 is the longer version.

Which number to trust for what

Use the private address to talk to a printer, a NAS, or the router admin page while you are on that LAN. Use the public address when a website, a support agent, or an allow-list asks for “your IP”. Use the router’s WAN address, compared with the public one, when you are deciding whether the ISP is sharing. Do not paste a 192.168 address into a service that runs somewhere else. It will either be rejected or, worse, interpreted as some other network’s private address.