The main difference between IPv4 and IPv6 is the addressing scheme and capacity. IPv4 uses 32-bit addresses, allowing for approximately 4.3 billion unique addresses, which has become insufficient due to the growth of the Internet and connected devices. In contrast, IPv6 employs a 128-bit address space, significantly expanding the number of possible addresses to an estimated 340 undecillion (3.4 x 10^38). This vast increase not only accommodates future growth but also simplifies address allocation and enhances routing efficiency, improving overall network functionality and sustainability.