The Internet Protocol: IPV4 Packets
Now that we have clarified the allocation of IPv4 addresses and the utilization of the longest prefix match to forward IPv4 packets, we can have a more detailed look at IPv4 by starting with the format of the IPv4 packets.
The IPv4 packet format was defined in RFC 791. Apart from a few clarifications and some backward compatibility changes, the IPv4 packet format did not change significantly since the publication of RFC 791. All IPv4 packets use a 20-byte header as shown below. Some IPv4 packets contain an optional header extension that is described later.
IPv4 Packet Header
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