It would be much simpler to use the host filter in the options to block all IP's except for your private network.
For example, add this to the banned hosts: *.*.*.*
And your private network: e.g. 10.*.*.* to the allowed hosts.

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Morgens ess ich Cornflakes und abends ess ich Brot
Und wenn ich lang genug gelebt hab, dann sterb ich und bin tot

--Fischmob