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The browser can't know what's an external or internal address (not that even we humans have a meaningful definition, not really a compatible concept with current networking)

This is one of the reasons network level controls are problematic and the solution is being reinvented as "zero trust networking".

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But it does know, 192.168.x.x is a reserved private network address.

RFC1918 space and cgn nat space etc are ambiguous addresses (vs unique like global ip space), but can be external or internal from your pov depending on circumstances.

Replying to myself with an addition: RFC1918 addresses are strictly worse wrt making up network level policy than normal IP addresses, you can apply the same rules for normal addresses on your network with the advantage that you know what they mean. (Good reason to prefer IPv6 too)

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