IP addresses

An IPv4 address is 32 bits long, which allows 4,294,967,296 addresses. The free pool ran out on 3 February 2011.

A common belief is that each device has one IP address for life, like a serial number printed inside it. It does not. The same phone gets one address on home Wi-Fi, a different one on mobile data, and may get a new one tomorrow. To see why addresses come and go, start with what an address is for.

Every packet carries two addresses in its header: where it came from and where it is going. These are IP addresses. A router reads the destination address, and nothing else, to decide where a packet goes next. So the way addresses are built shapes how the whole internet works.

Why no part is bigger than 255

The version most people still meet is IPv4, set out in RFC 791 in 1981. An IPv4 address is 32 bits. To make it readable, it is split into four groups of 8 bits, each written as a decimal number from 0 to 255, with dots in between, as in 192.168.0.12. Eight bits hold 256 patterns, so 255 is the top. All together there are 2 to the power 32 addresses: 4,294,967,296, a little under 4.3 billion.

The front of an address names the network

An address has two parts. The front part, the prefix, names a network, and the rest picks one device on it. The prefix length goes after a slash. In 192.168.0.0/24 the first 24 bits are fixed, which leaves 8 bits, or 256 addresses, for that network. This is what lets a router keep a single rule such as "everything starting 198.51.100 goes that way" instead of a rule for every device.

Why the number on a home laptop starts with 192.168

RFC 1918 sets aside three blocks for private networks: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. Anyone may use them inside a home or an office, and routers on the public internet do not carry them. That is why a laptop at home probably has an address like 192.168.0.12, and why the neighbour's laptop may have exactly the same one. When the laptop talks to a server, the home router swaps the private source address for its own public address and changes the source port too, so replies can find their way back. This is network address translation (NAT). It lets a whole household share one public IPv4 address.

Who lends a device its address

So where does the laptop's address come from? When a device joins a network, it broadcasts a request, and the router answers with a free private address, the router's own address to use as the gateway, and the address of a DNS resolver. This exchange is DHCP (RFC 2131). The address is lent for a limited time and renewed while the device stays connected, so the same laptop can get a different number tomorrow. Join a school's Wi-Fi and the same exchange happens with that network's DHCP server. On mobile data the phone gets its address from the mobile network instead. The other end can change too, because a website can move to a new server. That is one reason we use names such as example.com, and the DNS unit shows how names become addresses.

One address means the same thing on every computer. The loopback address, 127.0.0.1, always means "this machine", so a program can use it to talk to another program on the same computer without anything leaving it. And many devices today hold an IPv4 address and an IPv6 address side by side, using whichever the other end supports.

Four billion addresses ran out

Four billion addresses sounded like plenty in 1981. On 3 February 2011 the Internet Assigned Numbers Authority (IANA) handed its last five blocks of IPv4 space to the five regional registries, and the central free pool was empty. The long-term answer is IPv6 (RFC 8200), whose addresses are 128 bits long. That gives 2 to the power 128 addresses, about 3.4 × 10³⁸. IPv6 addresses are written as eight groups of four hexadecimal digits separated by colons. Leading zeros in a group can be dropped, and one run of all-zero groups can be replaced by "::". So 2001:0db8:0000:0000:​0000:0000:0000:0020 shortens to 2001:db8::20.