An IP address is the number that lets one device find another on a network. Your router has one that faces the internet and hands out different ones to every phone, laptop and TV behind it. This section explains the kinds of IP addresses you will run into, which ones are private, which are special, and where to look up the one you need.
The two IP addresses every home has
Every connected home has at least two IP addresses working at once. The public IP is assigned by your internet provider to your router and is what every website, game server and email provider sees. It is unique across the whole internet. The private IP is assigned by your router to each device on your network, drawn from ranges reserved for exactly that purpose, and it is reused in millions of other homes at the same time without conflict.
The router sits between the two and translates, a process called network address translation (NAT). When your laptop at 192.168.1.23 asks for a web page, the router rewrites the request so it appears to come from your public IP, remembers who asked, and delivers the reply back to the right device.
Private IP address ranges
Three IPv4 ranges are set aside for private networks by RFC 1918, plus one more used by carriers. Addresses in these ranges are never routed on the public internet.
| Range | CIDR | Addresses | Typical use |
|---|---|---|---|
| 10.0.0.0 – 10.255.255.255 | 10.0.0.0/8 | 16,777,216 | Large corporate networks, some ISP routers (Xfinity, Telstra) |
| 172.16.0.0 – 172.31.255.255 | 172.16.0.0/12 | 1,048,576 | Docker, cloud VPCs, enterprise segments |
| 192.168.0.0 – 192.168.255.255 | 192.168.0.0/16 | 65,536 | Almost every home and small-office router |
| 100.64.0.0 – 100.127.255.255 | 100.64.0.0/10 | 4,194,304 | Carrier-grade NAT (shared address space, RFC 6598) |
If your router shows a public address starting with 100.64 through 100.127, your provider is using carrier-grade NAT and you share a real public IP with other customers. That is why port forwarding sometimes fails on mobile and fibre connections.
Router login IP addresses
The private address your router uses for itself is the default gateway, and it doubles as the address of the router’s admin page. The most common ones are 192.168.1.1, 192.168.0.1 and 10.0.0.1, but each manufacturer has its own habits. The router login section lists the default IP, username and password for every major brand and ISP.
Special and reserved addresses
Some addresses have fixed meanings and are never assigned to ordinary devices.
| Address | Name | What it does |
|---|---|---|
| 127.0.0.1 | Loopback (localhost) | Points back at your own device. Traffic never touches the network. IPv6 equivalent is ::1. |
| 0.0.0.0 | Unspecified / any | Means “no particular address”. A server listening on 0.0.0.0 accepts connections on every interface. |
| 255.255.255.255 | Limited broadcast | Reaches every device on the local network. Used by DHCP when a device has no address yet. |
| 169.254.x.x | Link-local (APIPA) | Self-assigned when no DHCP server answers. Seeing one usually means the router is unreachable. |
| 224.0.0.0 – 239.255.255.255 | Multicast | One-to-many delivery for streaming, discovery protocols and routing updates. |
| 192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24 | Documentation | Reserved for examples in manuals and articles, which is why they appear on this site. |
IPv4 versus IPv6
IPv4 addresses are four numbers from 0 to 255 separated by dots, such as 203.0.113.42. There are about 4.3 billion of them and the free pool ran out in 2011. IPv6 addresses are eight groups of hexadecimal digits separated by colons, such as 2001:db8::8a2e:370:7334, and there are effectively unlimited. Most modern connections carry both at once; the homepage tool shows which one your browser used to reach this site.
Subnet masks and CIDR
A subnet mask, or its shorthand CIDR prefix, tells a device which part of an address identifies the network and which part identifies the host. A home network of 192.168.1.0/24 (mask 255.255.255.0) has 254 usable addresses, from .1 to .254. The subnet calculator in the tools section works out network address, broadcast address and host count for any prefix.