Reserved & Special-Use IP Addresses
Special-purpose IP ranges you'll encounter in network design.
Beyond the private ranges of RFC 1918, several IPv4 blocks are reserved for specific purposes by the IANA: loopback (127.0.0.0/8), link-local or APIPA addressing (169.254.0.0/16), carrier-grade NAT (100.64.0.0/10), the TEST-NET documentation ranges, multicast, and the limited broadcast address.
Knowing these ranges prevents real mistakes, such as assigning a documentation address into production or treating an APIPA address as a valid client assignment. It also explains the addresses you meet every day: 127.0.0.1 is the loopback address of the machine you are on, and 127.0.0.1:80 refers to an HTTP service listening on that local machine.
| CIDR / Address | Name | RFC | Use |
|---|---|---|---|
| 0.0.0.0/8 | "This" network | RFC 1122 | Local host identification |
| 127.0.0.0/8 | Loopback | RFC 1122 | Local loopback (127.0.0.1) |
| 169.254.0.0/16 | Link-local / APIPA | RFC 3927 | Automatic addressing when DHCP fails |
| 100.64.0.0/10 | CGNAT / Shared Address Space | RFC 6598 | Carrier-grade NAT |
| 192.0.2.0/24 | TEST-NET-1 | RFC 5737 | Documentation and examples |
| 198.51.100.0/24 | TEST-NET-2 | RFC 5737 | Documentation and examples |
| 203.0.113.0/24 | TEST-NET-3 | RFC 5737 | Documentation and examples |
| 224.0.0.0/4 | Multicast | RFC 5771 | IPv4 multicast |
| 240.0.0.0/4 | Reserved (Class E) | RFC 1112 | Future use / experimental |
| 255.255.255.255 | Limited Broadcast | RFC 919 | Broadcast to local network |
Frequently Asked Questions
What is 127.0.0.1?
127.0.0.1 is the IPv4 loopback address, which always points back at the machine itself. Anything you send to it stays on the host and never touches the network, and the whole 127.0.0.0/8 block is reserved for loopback.
What does 127.0.0.1:80 mean?
It is the loopback address 127.0.0.1 followed by port 80, so it means an HTTP service listening on the local machine. Requests to that address are answered by the host itself, which is useful when testing a web server before exposing it to the network.
Is 127.0.0.1 the same as localhost?
Yes. The name localhost resolves to 127.0.0.1, and to ::1 on IPv6, through the hosts file, so the two are interchangeable. Using the name keeps configuration readable, while the address avoids any dependency on name resolution.
What is 169.254.0.0/16 used for?
It is the link-local range your operating system uses for Automatic Private IP Addressing when DHCP fails. A device showing a 169.254 address has not received a lease, so treat that address as a symptom rather than a working configuration.
What is 100.64.0.0/10?
That is the shared address space carrier-grade NAT uses, defined in RFC 6598. It looks and behaves like private space and is not routable on the public internet, but it is a separate block from the RFC 1918 private ranges.
What are the TEST-NET documentation addresses?
RFC 5737 reserves 192.0.2.0/24, 198.51.100.0/24, and 203.0.113.0/24 for documentation and examples. They are safe to print in guides and sample configurations, and they must never be assigned to real devices.
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