Recently we did a POC for one of our client and during this POC we demonstrated the integration between MS CRM 2013 online with well-known billing software. The billing software had a prerequisite to store IP address of the servers from where we are consuming their API to communicate with their system. We found this KP where all the MS CRM 2013 online servers IP address are listed based on the region, where your MS CRM 2013 online organization is hosted for example:
Hope it will help someone!!!