which ipv6 address notation is valid

IPv6 addresses are 128 bits and represented as eight groups of four hexadecimal digits each, each group representing 16 bits. The address can be written in both lowercase and uppercase. The preferred format for writing an IPv6 address is x: x: x: x: x: x: x: x, where each “x” is the group of four hexadecimal digits and each group contains 16 bits. The term used for a group is a hextet.  So each “x” is a single hextet, 16 bits or four hexadecimal digits.

The range of IPv6 addresses are 0000:0000:0000:0000:0000:0000:0000:0000 to FFFF: FFFF: FFFF: FFFF: FFFF: FFFF: FFFF: FFFF. This expression is in hexadecimal. If we convert one hextet into binary, it should be 16 bits. For example, we have a hextet “0000” that is equal to 0000000000000000 (16 time 0s), and hextet “FFFF” is equal to 1111111111111111 (16 tim1 1s). The following is the primary relationship table of binary, decimal, and hexadecimal.

Decimal	Binary	Hexadecimal
0	0000	0
1	0001	1
2	0010	2
3	0011	3
4	0100	4
5	0101	5
6	0110	6
7	0111	7
8	1000	8
9	1001	9
10	1010	A
11	1011	B
12	1100	C
13	1101	D
14	1110	E
15	1111	F

The ideal method of representing IPv6 addresses is groups of eight hextet (32 hexadecimal digits). However, there are two rules we can apply to reduce the number of digits needed to represent an IPv6 address. The preferred format for representing an IPv6 address is the following.

2001 : 0000 : 0000 : 1111 : 1234 : 1000 : A000 : 0100

2001 : 0DA1 : B111 : 0000 : 0000 : ABCD : 0BCD : 1245

FE80 : 0000 : 2BCD : 0000 : 1234 : 4567 : 89AB : CDEF

FE80 : 8BAB : 0000 : 0000 : 0000 : 0000 : 0000 : 0123

FF02 : 0000 : 0000 : 0000 : 0000 : 0000 : 0000 : 0001

FE02 : 0000 : ABBB: 0000 : 0000 : 0001 : FF00 : 0200

0000 : 0000 : 0000 : 0000 : 0000 : 0000 : 0000 : 0001

0000 : 0000 : 0000 : 0000 : 0000 : 0000 : 0000 : 0000

Omitting Leading 0s

Omitting leading 0 is the first rule to reduce the notation of the IPv6 address. Following are the examples to omit any leading 0s (zeros) in any 16-bit section or hextet:

• 0100 can be represented as 100
• 0DA1 can be represented as DA1
• 0123 can be represented as 123
• 0000 can be represented as 0

This rule only omits leading 0s, not trailing 0s. Otherwise, the address would be ambiguous. The table below shows examples of omitting leading 0s from the IPv6 address.

Omit All 0 Segments

The second rule reduces the notation of IPv6 addresses using a double colon (::) that can replace any single, contiguous string of one or more hextet containing all 0s. The double colon (::) can only be used once within an address; otherwise, there would be more than one possible resulting address. For example, if we have an IPv6 address FE02: 0000 : ABBB: 0000: 0000: 0001: FF00: 0200 and we apply the technique like FE02:: ABBB:: 1: FF00: 200. It is not correct. When used with the omitting leading 0s technique, the notation of the IPv6 address can often be greatly reduced. This is commonly known as the compressed format.

Prefix Length Notation

The most left bits of the IPv6 address and the network bits length represented in CIDR format are known as the network prefix. The prefixes in IPv6 are used similarly to the subnet mask of IPv4 addresses. In IPv6, we use a notation similar to the CIDR mask representation in IPv4. The notation values are between 1 and 128 to represent the network bits.

For example, in IPv6 address 2001: ABC8: 1000: 000C: 0000: 0000: 0000: 0001/ 64, 2001: ABC8: 1000: 000C::/64 represents the network prefix and the range for this IP network is from 2001: ABC8: 1000: 000C: 0000: 0000: 0000: 0001/64 to 2001: ABC8: 1000: 000C: ffff: ffff: ffff: ffff/64.

The first three hextet (48 bits ) 2001:ABC8:1000 are the IP version 6 global routing prefix, and the next 16 bits(hextet) “000C ” is used for internal subnetting within an organization, and the last 64 bits are used for internal hosts of the network. The length of the IPv6 prefix is used to recognize how many bits of a Global Unicast IP version 6 address are in the network.