Which statement correctly compares IPv4 and IPv6 headers in terms of address length and header structure?

Which statement correctly compares IPv4 and IPv6 headers in terms of address length and header structure?

• A. IPv4 headers typically 20 bytes minimum with 32-bit addresses; IPv6 headers are fixed at 40 bytes with 128-bit addresses; IPv6 uses extension headers to carry options.
• B. IPv4 uses 128-bit addresses and a 40-byte header.
• C. IPv6 headers are variable length and do not have extension headers.
• D. IPv4 headers are 60 bytes minimum and use 64-bit addresses.

Answer: (A)

The key idea is how address length and header structure differ between IPv4 and IPv6. IPv4 uses 32-bit addresses, and its header can be as small as 20 bytes. The IPv4 header length can grow up to 60 bytes to accommodate options, but the minimum is 20. IPv6, on the other hand, uses 128-bit addresses to support a vastly larger address space, and its base header is fixed at 40 bytes. If additional information is needed, IPv6 doesn’t pack options into the main header; instead, it uses separate extension headers to carry options or additional features. This combination—short, fixed-size base header with a large address space and extension headers for options—matches the stated description. The other choices misstate the address sizes, header sizes, or how options are carried.

The key idea is how address length and header structure differ between IPv4 and IPv6. IPv4 uses 32-bit addresses, and its header can be as small as 20 bytes. The IPv4 header length can grow up to 60 bytes to accommodate options, but the minimum is 20. IPv6, on the other hand, uses 128-bit addresses to support a vastly larger address space, and its base header is fixed at 40 bytes. If additional information is needed, IPv6 doesn’t pack options into the main header; instead, it uses separate extension headers to carry options or additional features. This combination—short, fixed-size base header with a large address space and extension headers for options—matches the stated description. The other choices misstate the address sizes, header sizes, or how options are carried.