Which IP header field specifies the length of the entire datagram in bytes?

• A. IP Version
• B. IP Header Length
• C. Fragment Offset
• D. IP Datagram Length

Answer: (D)

The field that carries the total size of the IP datagram in bytes is the one that represents the entire packet length, header plus data. In IPv4, this is the 16‑bit field commonly called the Total Length, and in practice it’s referred to as the IP Datagram Length. It tells a router or a host how many bytes belong to this datagram, which is essential for determining the end of the packet and for validating or reassembling fragmented datagrams. The other fields have different roles: the Version field indicates IPv4 vs IPv6; the Header Length field shows how long the header is in 32‑bit words; and the Fragment Offset field indicates where a fragment starts within the original datagram, relevant when fragmentation has occurred.