Reserved IP Address Ranges Explained

The Internet is a commons. But not every address belongs to it.

When IANA carved up the IP address space, they set aside specific ranges that would never touch the public Internet. These aren't mistakes or leftovers—they're deliberate exceptions. Places where global uniqueness doesn't apply. Where millions of networks use identical addresses without collision. Where your computer talks to itself. Where devices quietly announce "something is broken."

Private Addresses: Sharing Through Context

Three ranges—10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16—solved one of networking's most elegant problems: how do you let everyone have their own network when there aren't enough addresses to go around?

You let them share.

Your home network uses 192.168.1.x. So does your neighbor's. So do millions of networks worldwide. There's no conflict because these addresses only mean something locally. Internet routers see packets with private addresses and discard them instantly—they're nonsense on the public Internet, meaningful only within their own walls.

The same address means different things in different networks. Context creates meaning.

This is why NAT exists. Your router translates "192.168.1.45 wants to talk to the world" into "my public IP wants to talk to the world." When the response comes back, it reverses the translation. Private addresses give your devices identity within your network. NAT gives them access beyond it.

127.0.0.1: The Mirror

When you send traffic to 127.0.0.1, it never leaves your computer. Packets loop back to their origin without touching a network card.

This is how your computer talks to itself. How a web browser connects to a development server running on the same machine. How programs test networking code without needing actual networks.

The entire 127.0.0.0/8 block is reserved for loopback, though only 127.0.0.1 sees regular use. "Localhost" isn't a place—it's a mirror. Software treats local processes as if they were remote services, and the illusion works because the network stack plays along.

169.254.x.x: The Distress Signal

If you see 169.254.x.x on your device, something failed.

This is APIPA—Automatic Private IP Addressing. Your computer's emergency plan when DHCP doesn't respond. Instead of staying silent, it picks a random address from 169.254.1.0 to 169.254.254.255 and checks if anyone else is using it.

With an APIPA address, you can talk to other devices on the same network segment. But you can't reach the Internet. Routers won't forward this traffic. It's local-only communication, and it's telling you: "I gave up waiting for proper configuration."

This is a diagnostic clue, not a solution. When you see 169.254.x.x, your DHCP server is down, unreachable, or misconfigured.

224.0.0.0/4: Multicast Groups

Multicast addresses don't identify devices—they identify groups.

One sender, many recipients, one transmission. A video stream sent to a multicast group reaches everyone who subscribed without the sender duplicating packets for each viewer. Unlike broadcast (which hits everyone whether they want it or not), multicast is selective. Devices join groups and receive only what they asked for.

Within this range, 224.0.0.0/24 handles local network control traffic that shouldn't escape your network. The 239.0.0.0/8 range is administratively scoped—organizations can use it internally without coordinating with anyone.

Other Reserved Ranges

0.0.0.0/8 means "this network" or "no address yet." You'll see 0.0.0.0 before DHCP completes, or in routing tables pointing to the default gateway.

100.64.0.0/10 exists for carrier-grade NAT. Your ISP gives you a 100.64.x.x address, then translates it again before reaching the Internet. NAT behind NAT.

240.0.0.0/4 is reserved for future use—the old "Class E" space, held back for experiments that never came.

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