What is the key difference between IPv4 and IPv6?

The correct answer highlights a fundamental characteristic of the two protocols: IPv4 utilizes 32-bit addresses, yielding a theoretical maximum of about 4.3 billion unique addresses. In contrast, IPv6 was developed to address the limitations of IPv4, particularly the exhaustion of available IP addresses due to the growth of the internet and connected devices. By using 128-bit addresses, IPv6 can accommodate an astronomical number of unique addresses—around 340 undecillion (3.4 x 10^38). This vast addressing capability is crucial for the expansion and sustainability of internet-connected devices.

The remaining options contain inaccuracies regarding the characteristics of both protocols. For instance, the first choice misrepresents the bit lengths of the addresses, while the third option incorrectly describes the types of addresses; both IPv4 and IPv6 can be represented in both numerical and textual (dotted-decimal or hexadecimal) formats. The last option incorrectly suggests that IPv4 is no longer in use, whereas it is still widely utilized despite the gradual transition to IPv6. This distinction is vital for understanding networking concepts and the evolution of internet protocols.