IPv4 address
203.0.113.42
Four numbers, 0–255, separated by dots. 32 bits, about 4.3 billion addresses in total.
IPv4 with a prefix
203.0.113.0/24
A block rather than one address. The /24 says the first 24 bits are the network, leaving 256 addresses.
IPv6 address
2001:db8::8a2e:370:7334
Eight groups of four hex digits. 128 bits. :: stands in for one run of zero groups.
What makes an address valid
An IPv4 address is four octets separated by dots, each between 0 and 255. That upper bound is the
single most common mistake: 256.1.1.1 is not a valid address, and neither is
192.168.1.256. Leading zeros are ambiguous too, because some software reads
010.1.1.1 as octal. Write 10.1.1.1 instead.
An IPv6 address is eight groups of four hexadecimal digits separated by colons. Leading zeros in a
group may be dropped, and exactly one run of all-zero groups may be collapsed to
::. So 2001:0db8:0000:0000:0000:8a2e:0370:7334 and
2001:db8::8a2e:370:7334 are the same address. Only one :: is allowed,
because two would be ambiguous about how many zero groups each one hides.
In a URL an IPv6 address goes in square brackets, like
https://[2001:db8::1]:8080/, so the colons in the address do not get confused
with the one before the port.
Addresses that are safe to publish
If you need an address for a manual, a slide, a screenshot or a test, use one of the ranges below rather than inventing one. Invented addresses almost always belong to somebody, and documentation that tells thousands of readers to connect to a real host is how that host ends up receiving traffic it never asked for.
This site follows its own advice: the portal screenshots on Tixx by Prefixx
use 203.0.113.0/24, which is why there is nothing blurred out in them.
| Range | Name | Defined by | What it is for |
|---|---|---|---|
| 0.0.0.0/8 | This network | RFC 1122 | Source address only, when the host does not yet know its own address. |
| 10.0.0.0/8 | Private use | RFC 1918 | Internal networks. Never routed on the public internet. |
| 100.64.0.0/10 | Carrier-grade NAT | RFC 6598 | Shared address space between a subscriber and their ISP. |
| 127.0.0.0/8 | Loopback | RFC 1122 | The host itself. 127.0.0.1 is the address almost everyone has typed. |
| 169.254.0.0/16 | Link-local | RFC 3927 | Self-assigned when DHCP fails. A symptom, rarely a choice. |
| 172.16.0.0/12 | Private use | RFC 1918 | Internal networks. The range people forget is only 16 /16s wide. |
| 192.0.0.0/24 | IETF protocol assignments | RFC 6890 | Reserved for protocol machinery. |
| 192.0.2.0/24 | Documentation (TEST-NET-1) | RFC 5737 | Safe to use in examples, manuals and screenshots. |
| 192.168.0.0/16 | Private use | RFC 1918 | The home-router range. 192.168.1.1 is a gateway, not a public address. |
| 198.18.0.0/15 | Benchmarking | RFC 2544 | Network device testing. Should never appear in production. |
| 198.51.100.0/24 | Documentation (TEST-NET-2) | RFC 5737 | Safe to use in examples. |
| 203.0.113.0/24 | Documentation (TEST-NET-3) | RFC 5737 | Safe to use in examples. The range this site uses in its own screenshots. |
| 224.0.0.0/4 | Multicast | RFC 5771 | One sender, many receivers. Not assignable to a host. |
| 240.0.0.0/4 | Reserved | RFC 1112 | Held back in 1989 and never released. Periodically proposed for reuse. |
| 255.255.255.255/32 | Limited broadcast | RFC 919 | Every host on the local segment. |
Private addresses are not yours to route
The three RFC 1918 ranges (10.0.0.0/8,
172.16.0.0/12 and 192.168.0.0/16) are free for anyone to use inside
their own network, and millions of networks use the same ones simultaneously. That is exactly
why they cannot be announced on the public internet: the address is not unique, so there is no
single place to deliver it.
This is the distinction that sends most people looking for address space in the first place. Private addresses solve numbering inside a network. Running mail, hosting a service, or appearing in someone else's firewall rule as a fixed source needs something different: globally unique space that a registry records against your organization.
IPv6 documentation space
The IPv6 equivalent of the ranges above is 2001:db8::/32, reserved by
RFC 3849. Every IPv6
example on this page uses it.
Common questions
What is an example of an IP address?
203.0.113.42 for IPv4 and 2001:db8::8a2e:370:7334 for IPv6. Both come
from ranges reserved for documentation, so neither points at a real machine.
Is 192.168.1.1 a real IP address?
It is a valid address, but a private one. It is usually the gateway on a home or office router and millions of networks use it at the same time. It cannot be reached from the internet.
What is my own IP address?
The what-is-my-IP tool shows the address you are connecting from, along with the network it belongs to and what the routing table says about it.