Which of the following is NOT a characteristic of IPv6?

The correct answer is based on the fundamental differences in the address structure between IPv4 and IPv6. IPv6 features a 128-bit address space, which is significantly larger than the 32-bit address space that IPv4 offers. This substantial increase allows for an almost limitless number of unique IP addresses, addressing a key limitation of IPv4 due to the exhaustion of available addresses.

While IPv6 does support enhanced security features through IPsec, and indeed improves routing efficiency with a simpler and more hierarchical address structure, it does not have a smaller address field than IPv4. In fact, the opposite is true; IPv6 has a much larger address field. Thus, stating that IPv6 has a smaller address field than IPv4 is incorrect, making this option the one that does not characterize IPv6 accurately.