IPv4 is the fourth version of the Internet Protocol (IP) and routes most internet traffic. It uses 32-bit addresses, allowing for over 4 billion devices to connect. Addresses are written in binary or dotted-decimal notation, with each part identifying the network or host. IPv4 addresses are divided into classes A-C that determine the portions for network vs host identification, with classes D-E being reserved. Issues with IPv4 include a limited address space and increasing routing tables as the internet grows.