The main differences between IPv4 and IPv6 lie in their addressing schemes. IPv4, which uses a 32-bit address format, offers about 4.3 billion unique addresses, which is insufficient for the growing number of internet-connected devices. In contrast, IPv6 employs a 128-bit address format, allowing for a virtually limitless number of unique addresses. Additionally, IPv6 includes enhanced features such as built-in security, improved routing, and better support for mobile devices, making it a necessary upgrade as the Internet evolves.