In the IPv4 header 'Flags' field, which bit is always reserved and set to 0?

In the IPv4 header, the 'Flags' field is a 3-bit field that is used to control fragmentation of the IP datagram. Each bit in this field has a specific purpose. The first bit (labeled as bit 0) is reserved and is always set to 0. This convention is in place to maintain backward compatibility and ensures the future extensibility of the IP protocol without affecting existing implementations.

The remaining bits in the Flags field have defined roles in relation to fragmentation: one bit indicates whether the datagram may be fragmented, and the other bit signals whether more fragments will follow the current fragment. However, the reserved bit is not used for these purposes and must remain unset (0) to avoid confusion in the protocol operation.

This is why the correct answer is the first bit being reserved and set to 0, while the other bits may have specific functions related to the overall process of fragmenting IP packets.