Layer 1 โ Region (country/ASN allowlist). OpenAI publishes a list of supported countries and compares your IP's geolocation country against it. If you resolve to mainland China, Hong Kong, Russia, Iran, etc., you're stopped here no matter how clean your IP is. This is the gate the basic "ChatGPT IP check" tools test.
Layer 2 โ Edge bot-management (the Cloudflare wall). OpenAI sits behind Cloudflare, which scores your IP before the request ever reaches OpenAI. Datacenter ranges, public VPN egress, Tor exits and bad-reputation IPs get challenged or hard-blocked here โ the infamous error 1020 / "Verify you are human" loop that never completes. Cloudflare WARP egress and many corporate Zero-Trust tunnels are themselves classified as datacenter and tripped here. Your region can be perfectly supported and you still never see the login box.
Layer 3 โ Account-level IP reputation & behavior. Once you reach OpenAI, your account is loosely bound to the IPs it logs in from. A datacenter or shared/blacklisted IP raises the abuse score; rapid country-hopping (US morning, Singapore noon) reads as account sharing or automation; a brand-new account on a heavily-recycled proxy IP is the classic combination behind "flagged for potential abuse" and "suspicious login."