Why do ISPs use CGNAT?
Short answer: there aren't enough IPv4 addresses for every home to have one. Carrier-grade NAT (CGNAT) lets an ISP put many customers behind one public IPv4 address. It's cheaper than buying more addresses, but it breaks inbound connections such as port forwarding. IPv6 is the long-term fix, because it has enough addresses for everyone.
1. IPv4 addresses ran out
IPv4 has about 4.3 billion addresses, far fewer than the number of devices online. In North America, the regional registry says its free pool of IPv4 addresses was depleted on 24 September 2015. Since then, an ISP that needs more IPv4 addresses has to qualify for a small special-case block, join a waiting list, or buy addresses from another organization on the transfer market. The European registry has also tightened its rules: in September 2026 it restricted opening extra member accounts, which some organizations had used to get more addresses.
When the internet standards body set aside a special range for CGNAT in 2012, it stated the reason plainly: IPv4 address space is nearly exhausted, but ISPs still have to support IPv4 growth until IPv6 is fully deployed (RFC 6598). That range, 100.64.0.0/10, is what your router shows as its WAN address when you're behind CGNAT. See the 100.64.0.0/10 explainer.
2. How sharing one address works
With CGNAT, your router gets a private address from the ISP. The ISP's NAT box translates your traffic to a shared public IPv4 address and gives your connections ports from that address. Because one infected or busy device could use up the shared ports for everyone, the standards expect ISPs to limit how many ports each customer can use (RFC 6269), and the CGNAT requirements document asks for fair sharing between subscribers (RFC 6888).
That's also why port forwarding stops working: incoming traffic reaches the ISP's shared address, and the NAT box has no rule to send it to your home. More in What is CGNAT?
3. Logging: who used that address?
With a shared address, the address alone no longer identifies a customer. To answer abuse reports or legal requests, an ISP has to know which customer used which port at what time. The standard on deterministic address mapping (RFC 7422) exists to cut the volume of those logs, by giving each customer a fixed block of ports that can be worked out instead of logged connection by connection. It's one of the extra costs ISPs take on with CGNAT.
4. Why some ISPs use it and others don't
- Address supply: older ISPs often hold enough IPv4 for their customers. Newer, fast-growing fiber, 5G home and satellite ISPs started after the free pool ran out, so they are more likely to share addresses. Compare the ISPs that use CGNAT with the ISPs without CGNAT.
- Network design: some ISPs run IPv6-only networks and carry IPv4 over them with methods like DS-Lite, 464XLAT or MAP-T, which also share IPv4. See no IPv4 on your router.
- Opt-outs: several ISPs keep a pool of public addresses for customers who ask, sometimes free, sometimes as a paid static IP. The ISP finder and static IP prices show which.
5. IPv6 gets around it
IPv6 has enough addresses to give every home its own range, so IPv6 traffic usually doesn't go through the CGNAT at all. Services and visitors that use IPv6 can reach a device in your home directly, if your router's IPv6 firewall allows it. Run the checker to see whether IPv6 works on your line.
What you can do
- Check whether you're behind CGNAT (and test a port or your NAT type).
- Look up your ISP's public IP, opt-out and static IP options, or a public IP guide for your ISP.
- Ask your ISP for a public IPv4, a static IP, or IPv6.
FAQ
Why does my ISP use CGNAT?
Because IPv4 addresses ran out. CGNAT lets an ISP share one public IPv4 address between many customers instead of buying more addresses, which are now only available through special-case blocks, waiting lists or the transfer market.
Is CGNAT bad?
For browsing and streaming it's usually fine. It breaks inbound connections, so port forwarding, hosting a server, some games' open NAT and remote access to home devices stop working over IPv4. Shared addresses can also bring extra CAPTCHAs or blocks.
Can I get off CGNAT?
Often, yes. Some ISPs move you to a public IPv4 for free on request, others sell a static IP, and some offer neither on home plans. Check your ISP's page on this site for its options and sources.
Does IPv6 replace CGNAT?
In the long run. IPv6 has enough addresses for every home, so IPv6 traffic usually skips the CGNAT. Until every site and network uses IPv6, ISPs still need some way to carry IPv4, and many share it.
This site sticks to ISP facts. It doesn't review or recommend third-party products or services.