The first IPv6 address in a network is assignable, but it is the router anycast address, one of several required anycast addresses. (Anycast addresses are unicast addresses that are assigned to multiple hosts.) This is defined by RFC 4291, IP Version 6 Addressing Architecture:
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.
Some devices, e.g. Cisco routers, require special handling of the router anycast address. Cisco requires the anycast keyword be placed on the address, but you also assign another address to the interface because the anycast address is shared among the routers.
There are also some reserved subnet anycast addresses:
They MUST NOT be used for unicast addresses assigned to any interface.
See RFC 2526, Reserved IPv6 Subnet Anycast Addresses:
2. Format of Reserved Subnet Anycast Addresses
Within each subnet, the highest 128 interface identifier values are
reserved for assignment as subnet anycast addresses.
The construction of a reserved subnet anycast address depends on the
type of IPv6 addresses used within the subnet, as indicated by the
format prefix in the addresses. In particular, for IPv6 address types
required to have 64-bit interface identifiers in EUI-64 format, the
universal/local bit MUST be set to 0 (local) in all reserved subnet
anycast addresses, to indicate that the interface identifier in the
address is not globally unique. IPv6 addresses of this type are
currently specified to be those having format prefixes 001 through
111, except for Multicast Addresses (1111 1111) [3].
Specifically, for IPv6 address types required to have to have 64-bit
interface identifiers in EUI-64 format, these reserved subnet anycast
addresses are constructed as follows:
| 64 bits | 57 bits | 7 bits |
+---------------------------------+------------------+------------+
| subnet prefix | 1111110111...111 | anycast ID |
+---------------------------------+------------------+------------+
| interface identifier field |
For other IPv6 address types (that is, with format prefixes other than
those listed above), the interface identifier is not in EUI-64 format
and may be other than 64 bits in length; these reserved subnet anycast
addresses for such address types are constructed as follows:
| n bits | 121-n bits | 7 bits |
+---------------------------------+------------------+------------+
| subnet prefix | 1111111...111111 | anycast ID |
+---------------------------------+------------------+------------+
| interface identifier field |
The subnet prefix here consists of all fields of the IPv6 address
except the interface identifier field. The interface identifier field
in these reserved subnet anycast addresses is formed from a 7-bit
anycast identifier ("anycast ID"), with the remaining (highest order)
bits filled with all one's; however, for interface identifiers in
EUI-64 format, the universal/local bit in the interface identifier
MUST be set to 0. The anycast identifier identifies a particular
reserved anycast address within the subnet prefix, from the set of
reserved subnet anycast addresses.
The motivation for reserving the highest addresses from each subnet
rather than the lowest addresses, is to avoid conflicting with some
existing official and unofficial uses of the low-numbered addresses in
a subnet. For example, these low-numbered addresses are often used for
the ends of a point-to-point link, for tunnel endpoints, for manually
configured unicast addresses when a hardware token is not available
for the network interface, and even for manually configured static
addresses for the routers on a link. Reserving only 128 values for
anycast identifiers (rather than perhaps 256) means that the minimum
possible size of interface identifiers in an IPv6 address is 8 bits
(including room in the subnet for unicast addresses as well as
reserved subnet anycast addresses), allowing the division between
subnet prefix and interface identifier in this case to be
byte-aligned.
As with all IPv6 anycast addresses [3], these reserved subnet anycast
addresses are allocated from the IPv6 unicast address space. All
reserved subnet anycast addresses as defined in this document are
reserved on all links, with all subnet prefixes. They MUST NOT be used
for unicast addresses assigned to any interface.
3. List of Reserved Subnet Anycast Addresses
Currently, the following anycast identifiers for these reserved subnet
anycast addresses are defined:
Decimal Hexadecimal Description
------- ----------- -----------
127 7F Reserved
126 7E Mobile IPv6 Home-Agents anycast [4]
0-125 00-7D Reserved
Additional anycast identifiers are expected to be defined in the
future.
There are a couple of exceptions to the above router and reserved subnet anycast addresses. One is the use of /128 addresses on interface such as loopback interfaces. The other exception is the use of /127 networks for point-to-point links that is allowed in RFC 6164, Using 127-Bit IPv6 Prefixes on Inter-Router Links:
On inter-router point-to-point links, it is useful, for security and
other reasons, to use 127-bit IPv6 prefixes. Such a practice parallels
the use of 31-bit prefixes in IPv4. This document specifies the
motivation for, and usages of, 127-bit IPv6 prefix lengths on
inter-router point-to-point links.
/125subnet, all your addresses are reserved anycast addresses that must not be assigned as host interface addresses. – Ron Maupin Nov 17 '19 at 03:48fd00::/8prefix, but the requirement is that the next 40 bits (Global ID) must be randomly chosen. This is detailed in RFC 4193, Unique Local IPv6 Unicast Addresses: "Locally assigned Global IDs MUST be generated with a pseudo-random algorithm consistent with [RANDOM]." Remember that the "U" in ULA is for "Unique," which is why the Global ID must be random to have a high probability of uniqueness. – Ron Maupin Nov 17 '19 at 04:09