Microsoft uses three categories for Office 365 network endpoints with different requirements: Optimize, Allow, and Default. For the Default category, in order to minimize network optimization change management work required, we are replacing IP addresses with URLs. This includes removing the IP addresses previously listed for Yammer.
Optimize: The vital few endpoints that require direct and local network egress and other network optimizations.
Allow: Approximately 100 endpoints that require proxy bypass to ensure that they are not passed through SSL decryption devices.
Default: The bulk of Office 365 endpoints that can be connected to through a default Internet gateway. These endpoints do not require specific IP Addresses to be published since network traffic to them is directed to the default Internet egress such as a proxy server.
Yammer is changing the IP addresses it uses as of October 22, 2018. If you used IP Addresses for Yammer endpoints, in order to prevent Yammer from being blocked in your organization, you need to remove the Yammer IP addresses and add *.yammer.com.