Peterson Davie 3.34

Question

Why does the Offset field in the $IP$ header measure the offset in $8$-byte units? (Hint: Recall that the Offset field is $13$ bits long.)

Answer 1

The IPv4 header allocates only 13 bits to the Offset field, but a packet’s length can be up to 2¹⁶-1.

In order to support fragmentation of a maximum-sized packet, we must count offsets in multiples of 2¹⁶⁻¹³= 2³bytes.