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Loopback address is a special IP number that is designated for the software loopback interface of a machine. The loopback interface has no hardware associated with it, and it is not physically connected to a network.
The loopback interface allows IT professionals to test IP software without worrying about broken or corrupted drivers or hardware.
IPv6 Loopback address is 0:0:0:0:0:0:0:1
It can also be expressed as ::1
IPv6 reserves two special addresses. They are 0:0:0:0:0:0:0:0 and 0:0:0:0:0:0:0:1. IPv6 uses 0:0:0:0:0:0:0:0 internal to the protocol implementation, so nodes cannot use it for their own communication purposes. IPv6 uses 0:0:0:0:0:0:0:1 as its Loopback address equivalent to 127.0.0.1 in IPv4
IPv6 header contains the following things:
1. Version - This field contains the version of the IP used in the packet. It is of 4-bit in IP version 6.
2. Traffic class - This is an 8-bits field determining the packet priority. Priority values subdivide into ranges: traffic where the source provides congestion control and non-congestion control traffic.
3. Flow label - This 20 bits specifies the QoS management. Originally created for giving real-time applications special service, but currently unused.
4. Payload length - This 16 bits determines the payload length in bytes. When cleared to zero, the option is a "Jumbo payload" (hop-by-hop).
5. Next header - This 8-bits field specifies the next encapsulated protocol. The values are compatible with those specified for the IPv4 protocol field.
6. Hop limit - This is an 8-bits field newly introduced in IPv6. It replaces the time to live field of IPv4.
7. Source Address - This 128 bits field determines the logical address of the host that is sending the packet.
8. Destination Address - This 128 bits field determines the logical address of the host that is receiving the packet