What is the maximum total size of an IPv4 packet, including its header?

The maximum total size of an IPv4 packet, including its header, is 65,535 bytes.

This limit comes from IPv4’s 16-bit Total Length field, which records the entire datagram size in octets. A 16-bit unsigned value can represent numbers from 0 through 65,535, setting the protocol’s theoretical maximum.

The limit includes both the IPv4 header and the transported data. Because the standard IPv4 header is normally 20 bytes, a packet using no optional header fields can carry at most 65,515 bytes of payload before fragmentation or another transport constraint applies.

This maximum is not the same as the usual Ethernet maximum transmission unit, commonly 1,500 bytes. Larger IPv4 datagrams may be fragmented into smaller pieces, although modern networks often avoid fragmentation through Path MTU Discovery. IPv6 uses a different approach: its base header has a 16-bit Payload Length field, while very large packets require a Jumbo Payload option.