Which padding values are used to pad IP options to a 4-byte boundary?

• A. Padding bits embedded in the header
• B. Zero-length padding values
• C. NOPs and EOLs
• D. End of option list markers

Answer: (C)

IP option padding is needed so the whole header length stays a multiple of 4 bytes, since the header is defined in 32-bit words. To pad without changing option meanings, two special one-byte options are used. The No-Operation option exists solely for alignment and carries no data or effect on option interpretation. The End of Option List option marks where the real options end; after it, any remaining bytes in the 4-byte boundary are simply padding. Using these two, the option area can be neatly padded to a 4-byte boundary, which is why NOPs and End of Option List are the padding values used.