Every device connected to the internet has a unique public address known as an IP address. The current IP address space (IPv4) consists of 4,294,967,296 (2^32) addresses, that is a 32bits number. Of these addresses some are reserved for private networks (about 18 million) and others for multicast (about 270 million), so in theory there can never be more than about 4 billion devices connected to the internet at any given moment. This is a limiting factor since there are about 7 billion people on earth who will eventually all have at least one phone and there are an uncountable number of other devices and Internet of Things (IoT) devices.

There is a new addressing schema being implemented called IPv6. An IPv6 address is a 128 bits number. That means that there are 340,282,366,920,938,463,463,374,607,431,768,211,456 possible addresses. That is approximately 340 undecillion or 340 billion billion billion billion possible IPv6 addresses, plenty of space for a long time to come.