How many bits are in the IPv4 header Version field, and what value identifies IPv4?

• A. 8 bits; the value 4 identifies IPv4
• B. 4 bits; the value 6 identifies IPv4
• C. 6 bits; the value 4 identifies IPv4
• D. 4 bits; the value 4 identifies IPv4

Answer: (D)

The Version field at the very start of the IPv4 header is 4 bits wide, and it carries the value that identifies the protocol version being used. For IPv4, that value is 4 (in binary, 0100). This simple 4-bit indicator lets devices know how to parse the rest of the header. It’s fixed at 4 for IPv4 (in contrast, IPv6 uses a different 4-bit value, namely 6). So the correct understanding is: 4 bits in the Version field, with the value 4 identifying IPv4.