Q & A on Sizes of IP headers and IP datagrams Q: How big can an IP datagram be? A: A datagram is limited to 64K octets (i.e., 2^16) because the header field that contains the total length is only 16 bits wide. Q: How big can an IP header be? A: A datagram header can be at most 60 octets. The header length is stored in a 4-bit field that gives the number of 32-bit values in the header, with each 32-bit value being 4 octets long. Thus, the maximum length is 15*4. Q: How big is a typical IP header? A: Because most datagrams do not contain IP options, the header stops after 20 octets. Q: Does an IP header grow larger when a datagram travels over an Ethernet? A: No. The datagram is encapsulated in an Ethernet frame for transmission, but the datagram header itself does not change. Of course, the frame contains an ethernet header in addition to the datagram (which travels in the frame payload area).