Streamlined Fragmentation fields moved out of base header IP options moved out of base header Header Checksum eliminated Header Length field eliminated Length field excludes IPv6 header Alignment changed from 32 to 64 bits Revised Time to Live Hop Limit Protocol Next Header Precedence and TOS Traffic Class Addresses increased 32 bits 128 bits Extended Flow Label field added The IPv6 header has 40 octets in contrast to the 20 octets in IPv4. IPv6 has a smaller number of fields, and the header is 64-bit aligned to enable fast processing by current processors. Address fields are four times larger than in IPv4. The IPv6 header contains these fields: Version: A 4-bit field, the same as in IPv4. It contains the number 6 instead of the number 4 for IPv4. Traffic Class: An 8-bit field similar to the type of service (ToS) field in IPv4. It tags the packet with a traffic class that it uses in differentiated services (DiffServ). These functionalities are the same for IPv6 and IPv4. Flow Label: A completely new 20-bit field. It tags a flow for the IP packets. It can be used for multilayer switching techniques and faster packet-switching performance. Payload Length: Similar to the Total Length field of IPv4. Next Header: The value of this field determines the type of information that follows the basic IPv6 header. It can be a transport-layer packet, such as TCP or UDP, or it can be an extension header. The next header field is similar to the Protocol field of IPv4. Hop Limit: This field specifies the maximum number of hops that an IP packet can traverse. Each hop or router decreases this field by one (similar to the Time to Live [TTL] field in IPv4). Because there is no checksum in the IPv6 header, the router can decrease the field without recomputing the checksum. On IPv4 routers the recomputation costs processing time. Source Address: This field has 16 octets or 128 bits. It identifies the source of the packet. Destination Address: This field has 16 octets or 128 bits. It identifies the destination of the packet. Extension Headers: The extension headers, if any, and the data portion of the packet follow the eight fields. The number of extension headers is not fixed, so the total length of the extension header chain is variable.
IPv6 Address Structure
• :: can be used once to represent a string of zeroes
ExampleInterface MAC 00-40-63-ca-9a-20IPv6 Interface ID (EUI-64) ::0040:63FF:FECA:9A20or ::40:63FF:FECA:9A20link local FE80::40:63FF:FECA:9A20
IPv4 and IPv6 Header Comparison IPv4 Header IPv6 Header Type ofVersion IHL Total Length Traffic Service Version Flow Label Class FragmentIdentification Flags Offset NextTime to Live Protocol Header Checksum Payload Length Hop Limit HeaderSource AddressDestination AddressOptions Padding Source Address Field’s Name Kept from IPv4 to IPv6Legend Fields Not Kept in IPv6 Name and Position Changed in IPv6 Destination Address New Field in IPv6
AAAA Records in DNS• iana.org and ipv6.net work too