You can ask IIS to restrict access to a resource by IP address from within the Web.config:
<location path="resources">
<system.webServer>
<security>
<ipSecurity allowUnlisted="false">
<clear/>
<add ipAddress="127.0.0.1"/>
</ipSecurity>
</security>
</system.webServer>
</location>
More info
EDIT: As Mike pointed it out in the comment below, this requires the IP and Domain Restrictions module to be installed. Thanks Mike!
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