In IPv4, addresses were created in classes. Classful IPv4 addresses
clearly defines the bits used for network prefixes and the bits used for
hosts on that network. To subnet in IPv4 we play with the default
classful netmask which allows us to borrow hosts bit to be used as
subnet bits. This results in multiple subnets but less hosts per
subnet. That is, when we borrow host bit to create a subnet that costs
us in lesser bit to be used for host addresses.
IPv6 addresses uses 128 bits to represent an address which includes
bits to be used for subnetting. Second half of the address (least
significant 64 bits) is always used for Hosts only. Therefore, there is
no compromise if we subnet the network. [Image: IPv6 Subnetting]16 Bits of subnet is equivalent to IPv4’s Class B Network. Using
these subnet bits an organization can have more 65 thousands of subnets
which is by far, more than enough.
Thus routing prefix is /64 and host portion is 64 bits. We though,
can further subnet the network beyond 16 bits of Subnet ID, borrowing
hosts bit but it is recommended that 64 bits should always be used for
hosts addresses because auto-configuration requires 64 bits.
IPv6 subnetting works on the same concept as Variable Length Subnet Masking in IPv4.
/48 prefix can be allocated to an organization providing it the
benefit of having up to /64 subnet prefixes, which is 65535
sub-networks, each having 264 hosts. A /64 prefix can be assigned to a
point-to-point connection where there are only two hosts (or IPv6
enabled devices) on a link.
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