An IP packet (sometimes referred to as a datagram) has a distinct and defined structure. In simple form, an IP packet is the IP packet data (which is nothing more than the segment that was passed down from the session layer) and the IP packet header, as shown in Figure 3-5.
Figure 3-5. Simple IP Packet Structure
IP Packet (20 to 65535 bytes in length}
IP Packet Header (20 or more by sea in length)
IP Packet Data
(variable length ranging from 1 to &SS1Sj bytes in most cases)
The IP packet header is typically 20 bytes in length (unless IP options are used, in which case the length may be variable up to a maximum length of 60 bytes) and contains the information that allows systems to determine how to process the corresponding IP packet data. The IP packet data is a variable length, ranging from 1 to 65515 bytes in length in most cases. Obviously, if the header is larger than 20 bytes, the IP packet data maximum size will be reduced in size accordingly. This provides for a minimum IP packet size of 21 bytes (20-byte header, 1-byte data) and a maximum IP packet size of 65535 bytes (20-byte header, 65515-byte data).
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