Special-Use IP Addresses

Why Some Ranges Are Reserved

The internet's authorities set aside certain blocks of the IPv4 space for specific purposes rather than handing them out as ordinary public addresses. The unifying property is that routers on the public internet are configured to never forward these addresses — they are confined to local or internal use. That single rule is what makes them so useful: because a private address can never travel the public internet, millions of separate networks can reuse the very same addresses without ever colliding. Your 192.168.1.1 and your neighbour's 192.168.1.1 coexist happily because neither ever leaves home.

Here is the at-a-glance reference, followed by what each range actually does:

The Three Private Ranges (RFC 1918)

The most important reservations are the three private ranges, defined in the document RFC 1918. These are the addresses used inside virtually every home and business network:

• 10.0.0.0/8 — over 16 million addresses; favoured by large organizations.
• 172.16.0.0/12 — about a million addresses across 172.16 through 172.31; common in mid-size networks.
• 192.168.0.0/16 — 65,536 addresses; the default for home routers, which is why 192.168.x.x is so familiar.

Devices on these addresses reach the internet through Network Address Translation, which swaps the private source address for the router's single public one on the way out. The deeper treatment of how private and public addressing relate is in private vs public IP addresses; this page is the range reference. The key fact to memorise is the trio above — any address starting 10., 192.168., or 172.16 through 172.31 is private and local.

Loopback: 127.0.0.0/8

The loopback range lets a device refer to itself. The famous 127.0.0.1 — "localhost" — is its most-used address, but the entire 127.0.0.0/8 block is reserved for the purpose. Traffic sent to a loopback address never touches a network card; the operating system loops it straight back internally. This is how you test a web server on your own machine, and how separate programs on one computer communicate over the network stack without any cabling involved. If you can reach 127.0.0.1 but nothing else, you have proven the local networking software works and the problem lies outward. See the loopback address for more.

Link-Local: 169.254.0.0/16 (APIPA)

An address in the 169.254 range is usually a symptom, not a choice. When a device is set to obtain its address automatically but cannot reach a DHCP server, it falls back to assigning itself a random address from this link-local range — a feature Windows calls APIPA (Automatic Private IP Addressing). Such an address works only for talking to other devices on the same physical link and is never routed further. In practice, spotting a 169.254.x.x address on a device that should have a normal one is a reliable sign that DHCP failed — the cable is unplugged, the DHCP server is down, or there is an address conflict.

Carrier-Grade NAT and Documentation Blocks

Two more reservations round out the set. The carrier-grade NAT range, 100.64.0.0/10, is shared space that internet providers use between their core network and customers when they have exhausted their supply of public IPv4 addresses — it lets an ISP place many subscribers behind a single public address, an arrangement explained in what is CGNAT. It is deliberately separate from the RFC 1918 private ranges so that it does not clash with the private addressing customers already use at home.

Finally, the documentation blocks — 192.0.2.0/24, 198.51.100.0/24, and 203.0.113.0/24 — exist purely for use in examples, manuals, and tutorials. They are guaranteed never to be assigned to a real host, so a writer can use them in sample configurations without accidentally pointing at someone's actual server. If you see one of these in a guide, it is a placeholder by design. Together with the all-zeros address 0.0.0.0 (meaning "this host" or "any address" depending on context), these reserved blocks make up the IPv4 addresses that, by rule, never behave like ordinary public ones.

Spotting Them in the Wild

Once these ranges are familiar, a glance at an address tells you a lot. A 10., 172.16–31., or 192.168. address is a private device behind NAT. A 127. address is the machine itself. A 169.254. address means automatic addressing kicked in because DHCP did not answer. A 100.64 through 100.127 address means your ISP is using carrier-grade NAT, which has real consequences for hosting servers or gaming. None of these will ever be the public face of a device on the internet — that always requires a genuine public address. For how addresses divide into networks and hosts in the first place, see IPv4 explained.

Frequently Asked Questions

What are the private IP address ranges?

Three ranges from RFC 1918: 10.0.0.0/8, 172.16.0.0/12 (172.16–31), and 192.168.0.0/16. They are reserved for private networks and never routed on the public internet, so countless networks reuse them simultaneously.

What is the 169.254 address range?

169.254.0.0/16 is link-local space, self-assigned when a device cannot reach a DHCP server — Windows calls it APIPA. A 169.254 address usually means DHCP failed. It works only on the local link and is never routed.

What is 127.0.0.1 used for?

It is the loopback address a device uses to refer to itself; traffic to it never leaves the machine. The whole 127.0.0.0/8 block is loopback, used to test local services and let software on one machine talk to other software on the same machine.

What is the 100.64.0.0 range?

100.64.0.0/10 is the carrier-grade NAT range, reserved for ISPs to use between their network and customers when short of public IPv4 addresses. It is shared space distinct from private and public addresses, letting an ISP put many customers behind one public address.

Why don't special-use addresses appear on the internet?

They are reserved for local or internal purposes, so public routers never forward them. That confinement is what makes it safe for millions of networks to reuse the same private addresses, which NAT translates before they reach the internet.