What are the fields in the IPv6 packet header and how do they compare to IPv4?

Question

Accepted Answer

Version, Traffic Class, Flow Label, Payload Length, Next Header, Hop Limit, and 128-bit Source/Destination addresses — simplified vs IPv4 by dropping the checksum and fragmentation fields.

* The IPv6 fixed header: Flow Label and Next Header are new; Hop Limit replaces TTL; the checksum is gone. *
IPv6 Header (fixed 40 bytes - simplified but not smaller):

Field
Size
Description

Version
4 bits
Set to 0110 (6) for IPv6

Traffic Class
8 bits
Equivalent to DS field for QoS

Flow Label
20 bits
Identifies packets belonging to the same flow for special handling

Payload Length
16 bits
Length of the data portion (not including header)

Next Header
8 bits
Identifies next protocol (TCP, UDP, ICMP, extension header)

Hop Limit
8 bits
Replaces TTL; decremented at each hop

Source IPv6 Address
128 bits
Sender's address

Destination IPv6 Address
128 bits
Receiver's address

Removed from IPv4:

Flag, Fragment Offset (fragmentation handled differently)
Header Checksum (removed to improve performance)

Go deeper:

RFC 8200 §3 — IPv6 Header Format — the authoritative field-by-field header definition.
 Wikipedia — IPv6 packet — fixed header fields plus the extension-header chain.

Field	Size	Description
Version	4 bits	Set to 0110 (6) for IPv6
Traffic Class	8 bits	Equivalent to DS field for QoS
Flow Label	20 bits	Identifies packets belonging to the same flow for special handling
Payload Length	16 bits	Length of the data portion (not including header)
Next Header	8 bits	Identifies next protocol (TCP, UDP, ICMP, extension header)
Hop Limit	8 bits	Replaces TTL; decremented at each hop
Source IPv6 Address	128 bits	Sender's address
Destination IPv6 Address	128 bits	Receiver's address