A new major milestone has been reached for Yammer
Yammer services will be moved to Microsoft Datacenters on May 16, 2015 – during the move process window there will be service disruption
Additionnaly if you restrict access to Office 365 services (and so including Yammer), the following IP range will be used by Yammer services after this move: 134.170.148.0/22
Check this post for all IP addresses used https://support.office.com/en-us/article/Office-365-URLs-and-IP-address-ranges-8548a211-3fe7-47cb-abb1-355ea5aa88a2?ui=en-US&rs=en-US&ad=US#BKMK_Yammer