limit IP's to the portal

Looking for considerations, recommendations or documentation on how one would limit IP’s from accessing the portal. My client would like to restrict access to the residing country IP's. What software/service for the portal to do I need to purchase for the restriction and maintenance of the subscription to ensure I always have a current list of ISPs IP’s

What are some of the recommeded solutions available for SAP?

Any advise is greatly apprectiated.

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