Reserved and private IP addresses

127.0.0.1 (localhost)

Block 127.0.0.0/8

The loopback address: traffic sent to it never leaves the device, it goes straight back to the same machine. The name localhost points to it. Seeing 127.0.0.1 in a log or a browser bar means software on that same computer made or answered the request. The whole 127.0.0.0/8 block is loopback, and IPv6 has a single loopback address, ::1.

0.0.0.0

Block 0.0.0.0/32

“This host on this network”: a device uses it as its source address before it has an address of its own, for example while asking DHCP for one. It is never a valid destination. A server configured to listen on 0.0.0.0 accepts connections on all of its IPv4 addresses, and 0.0.0.0/0 in a routing table is the default route, two conventions that reuse the same numbers.

192.168.1.1 and 192.168.0.1

Block 192.168.0.0/16

Private-use addresses: they work only inside a local network and are not routed on the internet. Many home routers use 192.168.1.1 or 192.168.0.1 as their own address, which is why typing it into a browser while connected to that network opens the router’s admin page. From outside the network the address reaches nothing.

10.0.0.1

Block 10.0.0.0/8

Part of the largest private-use block, 16.7 million addresses. Some ISP-supplied routers use 10.0.0.1 as their LAN address, and offices and cloud networks use 10.x.x.x internally. Like every private address, it is reachable only from inside its own network.

172.16.0.1

Block 172.16.0.0/12

Private-use, covering 172.16.0.0 to 172.31.255.255. Corporate networks use it, and Docker places containers on 172.17.0.0/16 by default. Addresses from 172.32.0.0 upward are ordinary public addresses.

169.254.x.x

Block 169.254.0.0/16

Link-local: a device gives itself an address here when nothing answers its DHCP request, so seeing 169.254.x.x on your own computer usually means it got no address from the router. Cloud platforms also serve instance metadata at 169.254.169.254, reachable only from inside the virtual machine.

100.64.x.x to 100.127.x.x

Block 100.64.0.0/10

Shared address space for carrier-grade NAT. If your router’s WAN address falls in this range, your provider puts several customers behind one public address, which is why sites see an IP different from the one on your router. Some VPN overlays, Tailscale among them, also number their devices from this block.

255.255.255.255

Block 255.255.255.255/32

Limited broadcast: a packet sent to it reaches every device on the local network segment and is never forwarded by a router.

192.0.2.x, 198.51.100.x, 203.0.113.x and 2001:db8::

Reserved for documentation and examples, so manuals and tutorials can show addresses that will never belong to anyone. An address from these blocks in real traffic is a configuration copied from an example.

fc00::/7, fd00::/8 and fe80::

Block fc00::/7

IPv6’s private ranges. Unique-local addresses (in practice fd00::/8) play the role of 192.168.x.x inside a site; fe80:: link-local addresses exist on every IPv6 interface and work only on its own link. Neither is routed on the internet.