The difference between IPv4 and IPv6 lies primarily in the address format and capacity. IPv4 uses a 32-bit address scheme, allowing for approximately 4.3 billion unique addresses, which have largely been exhausted due to the rapid growth of internet-connected devices. In contrast, IPv6 utilizes a 128-bit address system, vastly expanding the available address space to about 340 undecillion addresses. IPv6 not only accommodates more devices but also includes enhancements such as auto-configuration, improved security features, and better support for mobile devices, facilitating a more robust internet infrastructure.