IPv6 is an improved version of IPv4 with a larger 128-bit address size and 40-byte fixed header size, twice the size of IPv4's header. The IPv6 header contains information for routing, delivery, and addressing, with optional extension headers allowing additional non-essential fields to be included without increasing the fixed header size.

1. Internet Protocol Version 6 (IPv6) Header Format
• IP is a protocol that was defined in the TCP/IP model and used for
transmission of the packets from source to destination.
• IPv6 is an improved version of IPv4 and the header size of IPv6 is two
times greater than the IPv4 header size.
• The size of the IPv6 header is 40 bytes in length and it contains the
information required for routing and delivery.

2. Internet Protocol Version 6 (IPv6) Header
Format
• The size of the IPv6 address is four times greater than the size of
the IPv4 address but the IPv6 header size is only two times greater
than the IPv4 header size.
• There is one fixed size header and zero or more than zero optional or
extension headers in the IPv6 header. The fixed header keeps all the
information that is important for the router. Optional information is
kept in the extension header.

3. List of IPv6 Header Format Components
• There are two main parts to the IPv6 data packet that is header and
payload. The header of IPv6 is of a fixed length of 40 bytes which has
the following fields:

4. Internet Protocol Version 6 (IPv6) Header
Format
• Version
• Traffic Class
• Flow label:
• Payload Length (16-bits)
• Next Header (8-bits):
• Hop Limit (8-bits)
• Source Address (128 bits)
• Destination Address (128 bits)

5. IPv6 Fixed Header
• The size of the IPv6 fixed header is 40 bytes long and the IPv6 header
format consists of the following information:

6. IPv6 Fixed Header
Version (4-bits)
• It shows the version of the internet protocol we used, i.e. 0110
Traffic Class (8-bits)
• This is an 8-bit field in which 8 bits are divided into two parts. The most
significant 6-bit is for the type of service so that the router will get to know
about what services need to be provided to the given packet. And
for Explicit Congestion Notification (ECN), the least significant 2-bit is used.
Flow Label (20-bits)
• This 20-bit is required for maintaining the sequential flow of packets
related to a particular communication. This field is also helpful in avoiding
the reordering of packets. It is generally used for real or streaming media.

7. IPv6 Fixed Header
Payload Length (16-bits)
• This field is used to help the router know how much information is stored
in the payload of a particular packet.
Next Header (8-bits)
• This field is used to represent the type of extension header or if the
extension header is not present then it shows the Upper Layer PDU.
Hop Limit (8-bits)
• It works the same as that of TTL in IPv4. When it passes a hop or router its
value is decremented by 1. The packet is discarded when it reaches 0.
Source Address (128-bits)
• This field provides the address from where the packet originates.
Destination Address (128-bits)
• The destination address is the address of the packet's intended recipient.

8. IPv6 Extension Headers
• The fixed headers in IPv6 store only the information which is
necessary, instead of the information that is rarely used or not
needed. All this rarely used or not required information is stored in
the form of the extension header and placed between the fixed
header and the upper header.
• The headers that are arranged one after another in a linked list
format are known as extension headers.

9. IPv6 Extension Headers