Every computer/device on a network has a unique identifier. All devices connected to the internet use a protocol call TCP/IP where the IP is the unique identifier.
There are two versions, IPv4 and IPv6. IPv4 defines an IP as a 32 bit number allowing for 4,294,967,296 (2^32) addresses. As the amount of connected devices grew IPv6 was proposed which uses a 128 bit number which is 2^128 possible addresses, enough for the forseeable future.