The basic building block of IPv6 is the IPv6 packet header. Unlike IPv4, IPv6 uses a fixed length packet header. This improves processing efficiency of routing devices that the packet traverses through.
The illustration above shows the difference between an IPv4 header and an IPv6 header. You notice there isn’t a header length field in the IPv6 header. The header is always 40 octets (bytes) long.
Version (4 bits) – Always set to 0110 (6) representing version 6
Traffic Class (8 bits) – Most significant 6 bits are used for DSCP; 2 other bits used for ECN
Flow Label (20 bits) – First uses of this field are still emerging
Payload Length (16 bits) – size of payload in octets. Includes extension headers.
Next header (8 bits) – Indicates an IPv6 extension header or another header such as TCP or UDP header
Hop limit (8 bits) – Replaces TTL field. Each intermediate hop decrement this field by one. If it hits 0 the packet is dropped.
Source Address (128 bits) – Source IPv6 address
Destination Address (128 bits) – Destination IPv6 address
Now that you have an idea of what an IPv6 header looks like we will cover the 128-bit IPv6 address in our next article.