Currently, Microsoft provides a web page resource that provides the List of Office 365 IPs Required to be Whitelisted.
As of October 2, 2018, this resource will be deprecated (along with the XML document and RSS feed) in lieu of a REST-based web service. This new service will be used to publish Office 365 endpoints to make it easier for customers to evaluate, configure, and stay up to date with changes.
As a customer or a network perimeter device vendor, you can build against the new REST-based web service for the Office 365 IP address and FQDN entries.
- For the latest version of the Office 365 URLs and IP address ranges, use https://endpoints.office.com/version.
- For the data on the Office 365 URLs and IP address ranges page for firewalls and proxy servers, use https://endpoints.office.com/endpoints/worldwide.
To get the latest changes, use https://endpoints.office.com/changes/worldwide/0000000000.
Details: Announcing: Office 365 endpoint categories and Office 365 IP Address and URL web service
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