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IPv6 does not have broadcast, so is the last address usable? Besides it being long, is it bad practice to actually use it?

For example: A:B:C:D::0/64 starts with A:B:C:D:0000:0000:0000:0000 and ends with A:B:C:D:FFFF:FFFF:FFFF:FFFF.

Is A:B:C:D:FFFF:FFFF:FFFF:FFFF an address I can assign to a host?

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Any addresses in an IPv6 network can be assigned to a host.

There are a few special anycast addresses that should not be used.

RFC 4291, IP Version 6 Addressing Architecture defines the Router Anycast address:

2.6.1. Required Anycast Address

The Subnet-Router anycast address is predefined. Its format is as follows:

  |                         n bits                 |   128-n bits   |
  +------------------------------------------------+----------------+
  |                   subnet prefix                | 00000000000000 |
  +------------------------------------------------+----------------+

The "subnet prefix" in an anycast address is the prefix that identifies a specific link. This anycast address is syntactically the same as a unicast address for an interface on the link with the interface identifier set to zero.

Packets sent to the Subnet-Router anycast address will be delivered to one router on the subnet. All routers are required to support the Subnet-Router anycast addresses for the subnets to which they have interfaces.

The Subnet-Router anycast address is intended to be used for applications where a node needs to communicate with any one of the set of routers.

RFC 2526, Reserved IPv6 Subnet Anycast Addresses defines other anycast addresses that should not be used for host assignment:

2. Format of Reserved Subnet Anycast Addresses

Within each subnet, the highest 128 interface identifier values are reserved for assignment as subnet anycast addresses.

Remember that anycast addresses are unicast addresses that are assigned to multiple hosts, so in reality, every address in an IPv6 network can be used for host assignment.

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