Unfortunately there is no way to block websites at the network level (that I know of) as browsers and mobile phones have started using hardcoded DNS resolvers, so the utility of this is limited.
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Any network traffic which goes through a gateway under your control can be controlled. DNSSEC[0] can make this more difficult, true, but not impossible as content ultimately originates from an IPv4/IPv6 address and can be dropped by upstream network devices.
0 - https://en.wikipedia.org/wiki/Domain_Name_System_Security_Ex...