How does every device on the internet get an address?

Every device on the internet gets a number called an IP address, something like 192.168.1.1. It does two jobs at once: it says who the device is, and it says where in the network to find it, so a path to it can be worked out. Each packet of data carries two of these numbers in its header, the sender's and the destination's, which is how a reply knows its way back.

The address is built in two parts, like a street and a house number. The first bits name the network, and the remaining bits number the devices inside it. Organisations called regional Internet registries hand out blocks of networks, and internet providers pass them on.

The original version, IPv4, uses 32 bits, which allows 4,294,967,296 addresses. That looked like plenty. Vint Cerf, who co-created TCP/IP thinking it was an experiment, has admitted he thought 32 bits was enough. Yet people had anticipated the shortage since the late 1980s, and on 31 January 2011 the top-level pool of unallocated addresses ran out.

Two fixes keep things running. IPv6 uses 128-bit addresses, about 340 undecillion of them, though it does not talk to IPv4 directly, so both run side by side. And your home network hides behind a trick: private addresses, like the 192.168 range many home routers use, are not routed on the internet, so a single public address can serve a whole household.