If you open the configuration file /etc/squid/squid.conf, you will see the rules that allow IP addresses to connect to the proxy server like this:
... acl localnet src 192.168.0.0/16 ...
You can add a new ACL entry to allow a range of IP addresses to connect to your proxy server:
... acl localnet src 212.80.113.0/16 ...
Save the file.
WARNING: If you remove any ACL from the file, all IP addresses from that range will not be able to connect to the proxy server.
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sudo systemctl restart squid