IPv6 Header

0	1	2	3	4	5	6	7	8			9	10	11	12	13	14	15	16		17	18	19	20	21	22	23			24	25	26	27	28	29	30	31
Version				Traffic Class										Flow Label (20 bits)
0									1									2									3
Payload Length																		Next Header									Hop Length
4									5									6									7
Source Address (128 bits)
8										9								10										11
Source Address
12										13									14									15
Source Address
16										17									18									19
Source Address
20										21									22									23
Destination Address (128 bits)
24										25								26										27
Destination Address
28										29									30									31
Destination Address
32										33									34									35
Destination Address
36										37									38									39

Note: the standard IPv6 is 40 bytes long as opposed to 20 bytes for a v4 header (without options) also the v6 header is a lot simpler than is older brother and thus easier to understand. There are however, IPv6 extension headers that fit between the IPv6 header and next header, these include routing headers and fragment headers.

IPv6 Extension Headers
(RFC2460 IPv6 Spec)

An IPv6 packet can carry as many extension headers as it needs (Obviously within reason)
Example are below:-

IPv6 Header

Next Header = TCP

TCP Header + data

IPv6 Header

Next Header = Routing

Routing Header

Next Header = TCP

TCP Header + data

IPv6 Header

Next Header = Routing

Routing Header

Next Header = Fragment

Fragment Header

Next Header = TCP

TCP Header + data

Extension Header Order
The extension headers are not checked until it the destination identified. The following order is recommended in RFC 2460 and that they feature no more than once except for the destination options which can occur at most twice (once before a Routing header and once before the upper-layer header):

IPv6 header, Hop-by-Hop Options header, Destination Options header (note 1), Routing header, Fragment header

then

Authentication header (note 2), Encapsulating Security Payload header (note 2), Destination Options header (note 3) and then upper-layer header

note 1: for options to be processed by the first destination that appears in the IPv6 Destination Address field plus subsequent destinations listed in the Routing header.

note 2: additional recommendations regarding the relative order of the Authentication and Encapsulating Security Payload headers are given in [RFC-2406].

note 3: for options to be processed only by the final destination of the packet.