When the provider gives us the prefix, based on that prefix we must do subnetting, we can use IPv6 calculator for this purpose

https://www.calculadora-redes.com/ipv6.php

http://www.gestioip.net/cgi-bin/subnet_calculator.cgi

The distribution and assignment of IPv6 addresses in networks can be handled effectively using a variety of tools and methods, depending on the specific needs of the network and the scale of the deployment.

We present some of the recommended tools and approaches for IPv6 addressing management:

DHCPv6

This is a version of the Dynamic Host Configuration Protocol (DHCP) designed for IPv6. DHCPv6 can assign IP addresses, provide configuration information to clients automatically, and handle the renewal of IP addresses on the network. It is useful for larger networks where manual assignment is not practical.

SLAAC (Stateless Address Autoconfiguration)

SLAAC allows devices on the network to automatically configure themselves with IPv6 addresses without the need for a DHCP server. Each device generates its own address using its interface identifier and a prefix advertised by a local router. SLAAC is particularly useful for networks looking to simplify their address management without the overhead of maintaining a DHCP server.

RADVD (Router Advertisement Daemon)

It is a daemon for Unix/Linux systems that advertises the availability of IPv6 routes and network variables through the ICMPv6 protocol. It is crucial for setups that use SLAAC, as routers send advertisements that allow devices to configure their own IPv6 addresses.

Enterprise network managers

Tools like Cisco Prime Infrastructure, SolarWinds IP Address Manager, or Infoblox, which support IPv6, can help manage IPv6 address assignment centrally. These tools offer IP address space visualization, monitoring, and management and can integrate with DHCPv6 and DNS.

Custom scripts and automation

For networks with very specific needs, developing custom scripts or using automation platforms such as Ansible, Puppet, or Chef can help customize IPv6 address assignment and management to the exact network requirements.

RIPE NCC's IP Analyzer

Tools provided by Internet registries such as RIPE NCC include IP analyzers that can help plan and verify IPv6 subnet assignments to ensure they are used efficiently.

Choose the tool or combination of tools that best suits the size of your network, your technical requirements, and your available resources. Effective IPv6 address space management is crucial to the scalability and security of modern networks.