Carrier-Grade NAT (CGNAT) applies NAT at the ISP level, not just at your home router.
How it works:
- ISP assigns private addresses to customer routers (often 100.64.0.0/10)
- ISP's CGNAT device translates to public IPs
- Your home NAT translates again (double NAT)
Why ISPs use it:
- IPv4 addresses are exhausted
- One public IP can serve hundreds of customers
Problems:
- Double NAT makes port forwarding impossible
- Hosting from home becomes difficult
- All customers share IP reputation
CGNAT is common in mobile networks and residential ISPs.