For environments where websites/services have to be explicitly whitelisted in the firewall, the ports are 80 (HTTP) and 443 (HTTPS) and the full list of MTCaptcha Plugin & API service IPs are:
52.214.217.96
35.165.137.56
13.228.176.242
13.234.26.164
18.228.103.117
18.162.183.0
47.52.173.54
8.210.253.57
8.217.100.38
MTCaptcha Javascript Plugin uses a redundant dual-domain dual-network design which allows it to be delivered via both high performance dynamic CDN edge nodes as well as the more manageable and persistent static edge nodes IP’ed above.