[Edit, 7/7/16: Updating this post with the current range of IP addresses.]

Recently Authorize.Net migrated to a new data center to further enhance the performance and security of our platform. As part of the new platform changes, Relay Response and Silent Post data will now originate at these IP addresses:

198.241.168.60 
198.241.162.104

198.241.206.38

198.241.207.38

If you have a firewall that blocks inbound Internet traffic and use Relay Response or Silent Post, please add these IP addresses to your whitelist.