Configuring your firewall for Trello
You can configure your firewall to allow Trello as a trusted source by following these guidelines. Because Trello’s IP addresses can change frequently and without notice, we’d suggest that you use our site’s domain names instead of a particular site IP address.

First, please configure host names to recognize any sub-domain of:

* (paid accounts only)

In addition, these are the domains for the external resources for Trello: + (for SSO (paid accounts only)) (for dropbox attachments) (for attachments) (for one drive attachments)

via Configuring your firewall for Trello – Trello Help