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Unless EnableMediaRouter is set to Disabled, setting MediaRouterCastAllowAllIPs to Enabled connects Google Cast to Cast devices on all IP addresses, not just RFC1918/RFC4193 private addresses.
Setting the policy to Disabled connects Google Cast to Cast devices only on RFC1918/RFC4193.
Leaving the policy unset connects Google Cast to Cast devices only on RFC1918/RFC4193, unless the CastAllowAllIPs feature is turned on.