What Is a “CGNAT Provider”?

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A CGNAT provider is an Internet Service Provider (ISP) that employs Carrier-Grade NAT (CGNAT, also called Large-Scale NAT or LSN) in its network. Rather than assigning every customer a unique public IPv4 address, the provider gives customers private IPv4 addresses (or shares a limited public address pool) and uses NAT in their carrier network to translate many users’ traffic onto fewer public IPv4 addresses.

In other words: your home router might already be doing NAT (translating your devices’ private IPs to one “WAN / public” address). With CGNAT, the ISP inserts an additional NAT layer in their backbone, so there’s another translation before traffic reaches the public Internet.

The main motivation is IPv4 address exhaustion: public IPv4 addresses are limited, and CGNAT is a workaround to stretch usage of remaining addresses. Wikipedia+2A10 Networks+2

How CGNAT Works (Technically)

  • Address Sharing
    Many customers share a smaller pool of public IPv4 addresses. Each customer gets a private-range address (or a shared address) which is then translated by the ISP’s NAT box to one of the public addresses.

  • Double (or Triple) NATs
    Because the customer’s router already typically does NAT, traffic might pass through two or even three NAT layers:

    1. Device → home router NAT

    2. Home router → ISP’s CGNAT

    3. (Potentially) ISP’s private network NAT to public Internet
      This is sometimes referred to as NAT444 (customer private → carrier private → public)
      Alternatively, in a dual-stack or DS-Lite configuration, IPv6 may be used internally and reduce one NAT hop.

  • Port & Session Limits
    Because many users share one public address, each user may only get a limited number of source ports (and sessions) assigned. High port usage or long-lived connections may run into constraints.

Why ISPs Use CGNAT (Advantages & Motivations)

  1. IPv4 Address Conservation
    The primary driver is to mitigate the shortage of IPv4 addresses. By sharing public IPs among users, ISPs need fewer public addresses.

  2. Cost Efficiency
    Buying or leasing additional IPv4 space is expensive. CGNAT lets ISPs delay or reduce those costs.

  3. Transitional Strategy for IPv6
    As full transition to IPv6 is slow, CGNAT is seen as an intermediate step. Some CGNAT deployments coexist with IPv6 to ease the migration.

  4. Simplified Management of Address Space
    The ISP retains more control over IP allocation, routing, and security centrally, rather than distributing public IPs to many customers.

Challenges, Drawbacks & Impacts

While CGNAT is a practical workaround, it is not without issues. Below are some of its drawbacks:

Issue Description
Broken End-to-End Connectivity Hosting servers, peer-to-peer (P2P) services, remote access, and port forwarding become difficult or impossible because you can’t map a public port back to your internal host.
Increased Latency & Complexity Extra translation layer can add delay and complexity in path routing.
Port Exhaustion If many users congest a single public IP, you might run out of available ports, degrading performance for heavy users.
Logging & Accountability With shared addresses, tracking which user generated which traffic becomes harder (for logging, security, legal tracing) unless careful logging is implemented.
Protocol Breakage Some applications or protocols assume a public IPv4 and fail under double NAT (e.g., certain VPN, VoIP, gaming or P2P protocols).
User Experience Issues Some users notice weird connectivity problems, inability to forward ports, or difficulty running servers from home.

These concerns are documented in studies of CGNAT deployment and its effects on end-users

How to Detect If Your ISP Uses CGNAT

If you suspect your ISP is using CGNAT, here are some common methods to verify:

  1. Check Your WAN / External IP Address
    Log into your router and check the WAN IP shown there. Then, search “what is my IP” (via a public site).

    • If they differ, CGNAT is likely in play.

    • If your WAN IP falls within the “shared address space” block 100.64.0.0/10 (i.e. 100.64.0.1 to 100.127.255.254), that’s a strong indicator. If your WAN IP is a private address (e.g. 192.168.x.x, 10.x.x.x, 172.16.x.x), that’s a red flag.

  2. Test Port Forwarding / Hosting
    Try setting up port forwarding to a local device (e.g. a web server) and see whether it’s reachable externally.

  3. Traceroute / Hop Count
    Perform a traceroute or path analysis. If you see hops or segments indicating NAT boundaries or “private address” jumps, it might reveal a carrier NAT layer.

  4. Ask Your ISP
    Some ISPs openly state use of CGNAT in their terms or service plans.

  5. Use Diagnostic Tools
    Some network diagnostic tools (e.g. Netalyzr, the tests used in the academic study) can detect signs of carrier-grade NAT

Use Cases & Deployment Models

Here are a few ways CGNAT is often deployed:

  • NAT444 / Triple NAT
    Customer → private NAT → carrier NAT → public Internet. This is traditional CGNAT layering.

  • Dual-Stack Lite (DS-Lite)
    The ISP’s network is IPv6 internally; IPv4 traffic is tunneled and translated at a NAT box. This avoids one of the NAT layers.

  • Port Control Protocol (PCP) / NAT Control
    Some CGNAT systems support mechanisms for clients to request port mappings (somewhat akin to UPnP but for carrier NAT).

  • Hybrid IPv4/IPv6 approaches
    Some traffic or new customers may be IPv6-only internally, with translation for IPv4 use when needed.

Recommendations & Best Practices (For ISPs & Users)

For ISPs (or network architects):

  • Implement logging and mapping correlation so you can trace which customer used which port and time — essential for accountability and legal compliance.

  • Use port management to avoid port exhaustion (e.g. allocate per user quotas, dynamic port reuse).

  • Support PCP / NAT control protocols to allow users to request port mappings when necessary.

  • Provide optional public IPv4 or IPv6-only / IPv6-first plans for customers who need hostable services.

  • Plan for IPv6 transition, so CGNAT is a stop-gap, not a permanent crutch.

For users:

  • If you need to host services (games, servers, remote desktop), talk to your ISP about a static public IPv4 or IPv6 support.

  • Use tunnelling / relay services (e.g. Cloudflare Tunnel, NGROK, ZeroTier, Tailscale) to bypass NAT constraints. Many users note that even behind CGNAT, you can host things using such techniques.

  • Monitor your router’s WAN address and usage to detect any degradation.

  • If you’re tech-savvy, run periodic traceroutes or diagnostic tests to detect NAT boundaries.

The Future Outlook

While CGNAT is a practical stopgap, it’s not intended to be a permanent solution:

  • IPv4 exhaustion is well understood; the long-term path is full migration to IPv6.

  • As more services, applications, and users demand direct connectivity, double NAT will impose increasing friction.

  • Some next-generation network designs (e.g. IPv6 transition technologies, more IPv6 adoption) will reduce dependence on CGNAT.

  • Regulators, security, and legal systems often prefer clean address attribution, which CGNAT complicates.

In short, CGNAT providers are a symptom of the IPv4 overload era — useful but partial solutions until the Internet more fully embraces IPv6.