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:
52.214.217.9635.165.137.5613.228.176.24213.234.26.16418.228.103.11718.162.183.047.52.173.548.210.253.578.217.100.38MTCaptcha 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.