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:
*.braintreegateway.com (paid accounts only)
In addition, these are the domains for the external resources for Trello:
a.trellocdn.com (Trello's Content Delivery Network)
https://apis.google.com/ + https://accounts.google.com/o/oauth2 (for SSO (paid accounts only))
dropbox.com (for dropbox attachments)
app.box.com (for box.com attachments)
js.live.net (for one drive attachments)
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