Implement subnet support for clients

[DL6ER]

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10

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This PR implements subnet support for clients. Previously, only exact client / group associations have been possible, like 192.168.0.1. This PR allows for arbitrarily complex subnet configurations using the widely known Classless Inter-Domain Routing (CIDR) notation.

This allows users to specify "broad clients" such as

• 192.168.1.0/24 which will match al clients in the range 192.168.1.1 to 192.168.1.255 (256 devices),
• 10.8.0.0/16 will match all clients in the range 10.8.0.1 to 10.8.255.255 (65,536 devices),
• 192.168.100.0/22 represents the 1024 IPv4 addresses from 192.168.100.0 to 192.168.103.255.

This implementation is IPv6-aware and can be used for IPv6 subnets as well, such as the IPv6 block 2001:db8::/48 representing the block of IPv6 addresses from 2001:db8:0:0:0:0:0:0 to 2001:db8:0:ffff:ffff:ffff:ffff:ffff (1,208,925,819,614,629,174,706,176 devices).

Our implementation is more generic than what is written on the linked Wikipedia article as you can use any CIDR block (not only multiples of 4).

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/subnetting-doesnt-appear-to-work-for-groups/28813/25

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/netzwerk-in-clientliste-hinzufugen/34779/2