Every device that connects to the internet needs an IP address. But with the explosive growth of smartphones, IoT devices, and cloud services, the finite pool of IPv4 addresses has been exhausted. Internet Service Providers (ISPs) and telecom carriers needed a way to keep growing without immediately overhauling their infrastructure to IPv6.
The answer was Carrier-Grade Network Address Translation (CGNAT) — a large-scale version of NAT that allows thousands of subscribers to share a single public IPv4 address. CGNAT acts as a bridge, extending the usable life of IPv4 while giving carriers time to deploy IPv6.
Definition: What Is CGNAT?
Carrier-Grade NAT (CGNAT) is a network technology used by ISPs to conserve IPv4 addresses by mapping multiple private IP addresses from customer networks to a smaller pool of shared public IPv4 addresses.
Whereas traditional NAT (often in home routers) manages translation for a single household, CGNAT works at carrier scale, enabling thousands or even millions of users to access the internet with fewer public IPv4 addresses.
CGNAT is sometimes called Large Scale NAT (LSN) because of its ability to handle massive volumes of concurrent translations.
How CGNAT Works
At its core, CGNAT uses the same principles as NAT: translating between private and public IP addresses. The difference lies in scale, performance, and architecture.
- Private IP Addressing
- Subscribers use private IPv4 ranges (such as 10.0.0.0/8 or 192.168.0.0/16).
- These addresses are not routable on the public internet.
- Translation to Shared Public IPv4
- CGNAT devices translate private IPs into public IPs from a shared pool.
- Multiple users can share the same public IP, distinguished by unique port assignments.
- Port Address Translation (PAT)
- Since many users may map to a single public IP, CGNAT relies on port numbers to differentiate sessions.
- Each connection is tracked by a unique combination of private IP, public IP, and port.
- State Tracking
- CGNAT devices maintain massive state tables to track active connections.
- These systems must be highly performant to handle millions of concurrent flows.
- Logging and Compliance
- Because multiple users share an IP, ISPs must log translations to meet law enforcement and regulatory requirements.
Benefits of CGNAT
CGNAT is primarily about IPv4 survival, but it offers several operational benefits:
- IPv4 Extension: Prolongs the life of IPv4 by reducing the demand for unique public addresses.
- Scalability: Allows ISPs to continue onboarding customers without waiting for full IPv6 adoption.
- Cost Savings: Reduces the need to purchase scarce and expensive IPv4 addresses on secondary markets.
- Operational Flexibility: Provides a transitional path toward IPv6, letting carriers upgrade at their own pace.
- Service Continuity: Ensures customers can still connect to IPv4-based services while IPv6 matures.
Challenges of CGNAT
Despite its benefits, CGNAT introduces significant technical and user experience challenges:
- Traceability Issues: Multiple users sharing a public IP complicates attribution for law enforcement and security investigations.
- Application Breakage: Some applications, especially peer-to-peer services, VoIP, or online gaming, may not work correctly behind CGNAT.
- Performance Overhead: The need to maintain huge state tables can create latency and bottlenecks.
- Logging Complexity: Compliance requires detailed logging of NAT translations, which can be resource-intensive.
- Limited Future-Proofing: CGNAT is not a replacement for IPv6 — it only delays the inevitable transition.
Real-World Applications of CGNAT
CGNAT is most commonly deployed in:
- Telecommunications Providers: To serve millions of mobile subscribers without enough IPv4 addresses.
- Broadband ISPs: Supporting residential customers where IPv6 adoption is incomplete.
- IoT Networks: Allowing large-scale sensor and device deployments to connect over IPv4.
- Global Enterprises: Occasionally used internally when multiple sites must connect with limited public IP pools.
CGNAT vs. Related Technologies
- Traditional NAT: Handles address translation on a smaller scale (e.g., home routers). CGNAT scales this for entire ISP networks.
- IPv6: The long-term solution to address exhaustion. CGNAT helps bridge the gap until IPv6 is widely deployed.
- Dual Stack (IPv4/IPv6): Many ISPs use CGNAT alongside dual stack deployments to ease transition.
- NAT64/DNS64: Specific translation technologies that allow IPv6-only clients to reach IPv4 servers.
Industry Trends in CGNAT
Carrier-Grade NAT continues to evolve in response to growing connectivity demands.
- Increased IPv4 Scarcity: The secondary IPv4 market drives more carriers toward CGNAT adoption.
- Integration with Firewalls: Some CGNAT solutions now embed security features, filtering malicious traffic.
- Support for 5G and IoT: CGNAT is heavily used in mobile networks where IPv4 addresses are scarce.
- Transition to IPv6: Many providers position CGNAT as a stepping stone, running dual stack until IPv6 is dominant.
- Enhanced Logging: Vendors are improving logging systems to comply with regulatory requirements around attribution.
Best Practices for CGNAT Deployment
- Plan for Dual Stack: Don’t rely solely on CGNAT — combine it with IPv6 adoption.
- Invest in Logging Infrastructure: Ensure compliance by maintaining robust, searchable logs.
- Test Application Compatibility: Identify and address services that may break under CGNAT.
- Monitor Performance: Use network monitoring to ensure CGNAT does not become a bottleneck.
- Educate Customers: Provide documentation on how CGNAT may affect gaming, VoIP, or remote access.
Example: ISP Deployment
A regional broadband provider faced IPv4 exhaustion with only a limited pool of public addresses available. By deploying CGNAT:
- They mapped tens of thousands of customers to fewer than 1,000 public IPs.
- Logging systems ensured regulatory compliance.
- IPv6 was rolled out in parallel, giving customers dual-stack connectivity.
The result: continued growth without immediate dependence on expensive IPv4 purchases.
Related Solutions
Carrier-Grade NAT is a critical tool for extending IPv4 but works best as part of a broader connectivity and modernization strategy. SD-WAN can help optimize routing when CGNAT adds complexity, Global WAN Services provide private interconnection beyond IPv4 limits, and Cloud Connect enables enterprises to link directly to cloud providers without public address strain.
Explore related solutions that extend the value of CGNAT in modern enterprise and carrier networks:
