IPv4 (Internet Protocol version 4) is defined in RFC 791, published in 1981. The ARPANET planned its complete switch-over to TCP/IP for 1 January 1983 (RFC 801). An IPv4 address is a 32-bit number, usually written as four decimal numbers separated by dots — for example 203.0.113.42.
How an IPv4 address is written
The 32 bits are split into four 8-bit parts called octets, so each number is from 0 to 255:
203 .0 .113 .42
11001011 .00000000 .01110001 .00101010 # the same address in binary32 bits allow 232 = 4,294,967,296 different addresses — about 4.3 billion.
Why IPv4 ran out
On 3 February 2011, IANA allocated its last five /8 blocks to the five Regional Internet Registries, and “the free pool of available IPv4 addresses is now fully depleted” (ARIN announcement). Two techniques stretch the remaining addresses:
- NAT (Network Address Translation) — lets many devices on a private network share one public address (RFC 3022).
- Carrier-grade NAT (CGN) — ISPs share public addresses between customers; RFC 6598 reserves
100.64.0.0/10for this.
The long-term fix is IPv6.
Special IPv4 ranges
These ranges are reserved for special purposes (full list: IANA IPv4 Special-Purpose Address Registry):
| Range | Purpose | Defined in |
|---|---|---|
10.0.0.0/8 |
Private networks | RFC 1918 |
172.16.0.0/12 |
Private networks (172.16.0.0 – 172.31.255.255) | RFC 1918 |
192.168.0.0/16 |
Private networks | RFC 1918 |
100.64.0.0/10 |
Shared address space for carrier-grade NAT | RFC 6598 |
127.0.0.0/8 |
Loopback — 127.0.0.1 is “this computer” (localhost) |
RFC 1122 |
169.254.0.0/16 |
Link-local — self-assigned when no address is configured | RFC 3927 |
0.0.0.0 |
“This host on this network” — used before a host has an address | RFC 1122 |
255.255.255.255 |
Limited broadcast to the local network | RFC 919 |
192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24 |
Documentation and examples | RFC 5737 |
Tip: a 169.254.x.x address means the device configured itself because it didn't get an address from a DHCP server (RFC 3927) — try reconnecting to the network or restarting the router.
What does “/24” mean? (CIDR notation)
In CIDR notation (RFC 4632), the number after the slash is how many leading bits identify the network; the remaining bits identify hosts on it.
| CIDR | Subnet mask | Addresses |
|---|---|---|
/8 |
255.0.0.0 |
224 = 16,777,216 |
/16 |
255.255.0.0 |
216 = 65,536 |
/24 |
255.255.255.0 |
28 = 256 |
/32 |
255.255.255.255 |
1 — a single address |
So 192.168.1.0/24 covers 192.168.1.0 to 192.168.1.255. In a normal subnet the all-zeros and all-ones host addresses (.0 and .255 here) aren't assigned to devices (RFC 1122, §3.2.1.3), leaving 254.
How to see your IPv4 address
Your public IPv4 is shown on the home page, or from a terminal:
curl -4 https://forgotmyip.comFor your device's private address, see How to find your IP address.