This information was originally posted by Terry Lyons on his blog.

Feb 12 update from Terry's blog:

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POSTPONED: Additional Windows Live Messenger PIC/Federation IP Address

Reference my original blog on an Additional Windows Live Messenger PIC/Federation IP Address

Tomorrow’s planned added capacity has been postponed for technical reasons and to provide additional time for administrators to update their firewall settings if necessary to reflect the additional IP address.  The new date that this work is scheduled to go into place is Friday, March 26, 2010.  For organizations that have chosen to restrict this traffic to specific IP addresses, you must have the list in KB 897567 in place prior to Friday, March 26, 2010 or PIC connectivity with MSN will fail.

897567    Known issues that occur with public instant messaging and Communications Server
http://support.microsoft.com/default.aspx?scid=kb;EN-US;897567

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Notification

In an effort to provide enhanced capacity and service reliance, Windows Live Messenger will soon be adding an additional IP address used for PIC/Federation traffic.  Some organizations have chosen to restrict this type of traffic to specific IP addresses, as referenced in Microsoft KB 897567 (http://support.microsoft.com/kb/897567).  With this in mind we want to give you advanced notice of our intended change if you have configured your enterprise network in this manner.

Please ensure your enterprise firewall configuration is updated with the full list of Windows Live Messenger addresses below on or before Friday, February 12, 2010.  Windows Live Messenger will NOT enable the additional IP addresses until on or after February 12, 2010 (Pacific Time) to ensure your services will not be disrupted by this change.

IP address for Windows Live Messenger PIC/Federation:

64.4.9.181
64.4.9.245
64.4.50.110 (Additional new IP address)
65.54.52.53
65.54.52.245
65.54.227.249

For more information please reference Microsoft KB 897567, or for further assistance, please engage Microsoft Customer Support Services via http://support.microsoft.com/.