In the Enforce UI the configuration section for enabling Netblocks is intended for users to add external and routable IP addresses or ranges of addresses.

If a user adds a non-routable (non-public) Netblock the UI will present an error which reads "Please review the problems below:  Invalid IP address range".

For further reading about configuring Netblocks see the following articles:

Internal Senders

Enforce Self-Onboarding Guide

Configuration of Internal Sources