Some company networks/firewalls require whitelisting of particular domains in order to have them work for users on those networks. If you’re trying to do that, here’s the domains related to using CodePen:

// Main Domain
codepen.io
*.codepen.io

// Classic Editor Previews
cdpn.io

// Project Previews / Deployments
*.codepen.website

// 2.0 Editor Preview
codepen.dev
codepenusercontent.com

// 2.0 Editor Deployments
codepen.app