Which of the following is a characteristic of IPv4 addresses?

IPv4 addresses are characterized by a 32-bit address space. This means that each IPv4 address consists of 32 binary digits (bits), which allows for a total of approximately 4.3 billion unique addresses (specifically, 2^32 addresses). Each IPv4 address is typically represented in decimal format as four octets (e.g., 192.168.1.1), where each octet corresponds to 8 bits.

The other choices describe address spaces that do not apply to IPv4. The 64-bit address space is associated with IPv6, which is designed to accommodate a vastly larger number of devices and networks. A 128-bit address space is also part of IPv6 addressing, which allows for an even greater number of unique addresses than 64-bit. The 256-bit address space does not correspond to any commonly used internet protocol for addressing. Therefore, the correct characteristic of IPv4 addresses being a 32-bit address space is essential for understanding how IPv4 networks operate and is foundational knowledge for anyone preparing for the CompTIA A+ certification.