"Why Am I Banned?"
Sharing a public IP address means sharing its history and reputation. This is one of the most frustrating and least-understood consequences of CGNAT for end users.
How IP reputation works: Security systems, spam filters, and abuse databases track malicious activity by IP address. If an IP address sends spam, launches attacks, or violates terms of service, it gets flagged. Future requests from that IP are treated with suspicion or blocked outright.
Under CGNAT, a single public IP might represent 500 to 2,000 customers. If any one of them has malware, runs a spam operation, or violates a platform's terms of service, the entire pool of customers sharing that IP suffers the consequences.
Real-world scenarios:
Wikipedia editing blocks: Wikipedia blocks IP addresses used for vandalism. CGNAT IPs frequently appear on these blocklists because with thousands of users behind one IP, the statistical probability that at least one has vandalized an article is high. Legitimate users trying to contribute get the message: "Your IP address has been blocked."
Google CAPTCHA loops: Google's abuse detection sees thousands of searches per minute from a single IP and concludes it's a bot. Users behind CGNAT are subjected to frequent, sometimes endless CAPTCHA challenges.
Email blacklisting: If one customer behind a CGNAT IP runs a mail server or has a spam-sending botnet infection, the shared IP gets added to email blacklists (Spamhaus, SORBS, etc.). Every other customer's email sent through that IP gets rejected or marked as spam.
Rate limiting failures: APIs and web services implement rate limiting per IP address. Under CGNAT, 1,000 users share one IP, so the rate limit is effectively divided by 1,000. A service allowing 100 requests/minute per IP gives each CGNAT user a budget of 0.1 requests/minute before they're throttled.
Security logging and forensics: When an ISP or law enforcement needs to identify who performed a specific action at a specific time, they need both the IP address and the exact timestamp and port number used. Under CGNAT, the ISP must maintain logs mapping (public IP, public port, timestamp) → (customer account, private IP). These logs are large, expensive to store, and legally required in many jurisdictions. If the logs are incomplete or the timestamp is off by even a second, the wrong customer could be identified.
A common SRE/security interview question: "Your rate limiting is based on IP address. A customer complains they're being throttled despite low usage. How do you diagnose this?" The answer involves checking whether the customer is behind CGNAT — their IP may be shared with thousands of others who are consuming the rate limit budget. The fix is to move rate limiting to authenticated session tokens or API keys rather than IP addresses, which is the correct approach regardless of CGNAT.
Key Point: For platform engineers building services, the lesson is clear: never use IP address as a proxy for user identity or for per-user rate limiting. IP addresses are not unique identifiers under CGNAT. Use authenticated sessions, API keys, or user accounts for any per-user enforcement.