What is Carrier Grade NAT (CGNAT)

Carrier-Grade NAT, or simply CGNAT, is a large-scale version of Network Address Translation used by internet service providers to make IPv4 last a little longer. In simple terms, it lets thousands of users share a single public IPv4 address by assigning them different port ranges. As the pool of IPv4 addresses keeps shrinking and full IPv6 adoption remains a work in progress, CGNAT has become an essential part of modern telecom infrastructure.

In this article, we’ll explore what CGNAT actually does, why operators rely on it, how it works behind the scenes, and what advantages and limitations it brings.

At its core, CGNAT is a large-scale version of Network Address Translation designed for high-throughput carrier environments. While traditional NAT allows a home router to let several devices share one public IP address, CGNAT scales this principle to the level of internet service providers, enabling hundreds or even thousands of subscribers to share the same address simultaneously. This requires highly efficient algorithms to manage port translation, extensive NAT tables that can track millions of sessions, and mechanisms to ensure reliability under heavy loads.

In practice, CGNAT devices allocate port ranges to subscribers, monitor session lifetimes, and implement safeguards to prevent port exhaustion. Operators must also handle logging at scale to meet legal traceability requirements.

If you’ve spent any time reading about networking, you’ve probably noticed that Carrier-Grade NAT doesn’t always go by the same name. Depending on who’s talking — a network engineer, a standards document, or a vendor — the terminology changes slightly, even though they all mean more or less the same thing.

Here are the most common ways people refer to it:

• CGNAT — short for Carrier-Grade NAT.
• CGN — a shorter version, often used in technical specifications or standards.
• LSN (Large-Scale NAT) — the formal name used in many RFCs and white papers.
• Provider NAT — means NAT that runs inside the operator’s network, not in a home router.
• SP NAT — Service Provider NAT, another variation of the same concept.

In practice, these all describe the same idea — a way for many subscribers to share a limited number of IPv4 addresses, with translation happening inside the provider’s network.

Traditional NAT, typically implemented in home routers, performs a straightforward task: a single public IPv4 address is shared among several devices within one local network. It’s a simple one-to-many translation model.

Carrier-Grade NAT operates on a much larger scale. Instead of translating traffic for a few local devices, it maps many subscribers across multiple public IPv4 addresses simultaneously. Each session requires a unique port assignment, and every mapping must be logged for traceability and regulatory compliance.

The difference in scale introduces entirely new challenges. CGNAT systems often maintain tens of millions of concurrent sessions, with logging systems capable of processing and storing petabytes of data. Operating such infrastructure demands advanced mechanisms for monitoring, redundancy, and fault tolerance.

While traditional NAT serves as a convenient feature in residential networking, CGNAT functions as a core component of service provider infrastructure, ensuring IPv4 connectivity remains viable in large-scale networks.

The global pool of IPv4 addresses has long been exhausted. With only about 4.3 billion unique addresses ever available, the rapid growth of broadband connections, mobile subscribers, and IoT devices quickly consumed what remained. Each new customer or connected sensor still depends on that limited address space, which has now reached its capacity.

IPv6 was introduced as a long-term solution, providing an almost unlimited number of addresses. However, full adoption remains uneven. Many operators continue to run mixed IPv4/IPv6 environments, while parts of the ecosystem — from legacy hardware to certain applications — still lack native IPv6 support. Migrating to IPv6 at scale involves infrastructure upgrades, software updates, and operational retraining, which for many networks is a gradual, multi-year process.

Until that transition is complete, CGNAT serves as a practical bridge. By allowing thousands of users to share a single public IPv4 address, service providers can maintain connectivity without buying additional address blocks on the secondary market. This approach helps operators manage limited IPv4 resources efficiently, control costs, and ensure service continuity while IPv6 deployment continues to expand.

Carrier-Grade NAT functions by converting subscribers’ private IP addresses into shared public IPv4 addresses, distinguishing each connection through individual port numbers. For every active session, the system assigns a dedicated port or a defined range of ports and records these associations in a NAT table. Since a single provider may manage millions of concurrent data flows, these tables can become extremely large and must deliver high-speed lookups and continuous updates without delay.

Modern CGNAT solutions are designed for heavy-duty environments, sustaining throughput of hundreds of gigabits per second and processing hundreds of thousands of new translations each second.

Different architectures exist for different network contexts:

• NAT44 – the most common, translating IPv4-to-IPv4 at carrier scale.
• NAT64 – enables IPv6-only clients to reach IPv4 servers by converting protocols and addresses.
• DS-Lite – Dual-Stack Lite encapsulates IPv4 traffic over an IPv6 backbone before translation, allowing operators to migrate their cores to IPv6 while still supporting IPv4

Many of the issues once associated with Carrier-Grade NAT have been significantly reduced with the evolution of modern platforms. Earlier generations of CGNAT made inbound connectivity difficult, creating challenges for services such as hosting, peer-to-peer communication, and remote access. Contemporary solutions have largely overcome this by implementing endpoint-independent mapping, improved NAT traversal techniques, and flexible port management, which together make these scenarios much more dependable.

Performance concerns have also been addressed. In the past, applications sensitive to delay — such as VoIP, online gaming, or video conferencing — often suffered when passing through carrier-grade translation. Current CGNAT systems employ intelligent port allocation, hardware offloading, and dynamic load distribution to keep latency and jitter within acceptable limits. When properly configured, issues like port exhaustion or real-time traffic degradation are now rare.

From an operational perspective, visibility and traceability remain crucial. Because multiple users share the same public address, operators must correlate IPs, ports, and timestamps to identify individual connections. The Port Block Allocation (PBA) method, used in solutions such as NFWare CGNAT, simplifies this process by assigning subscribers entire port ranges instead of logging each session individually. This approach drastically reduces log volume while preserving full traceability. Even in jurisdictions that require long-term data retention, modern CGNAT platforms provide the scalability and resource management necessary to meet both regulatory and technical demands efficiently.

Carrier-Grade NAT plays a vital role in extending the usability of IPv4, but it is ultimately a temporary measure rather than a long-term fix. IPv6, built on a 128-bit addressing scheme, removes the problem of address scarcity altogether and restores the original end-to-end connectivity model of the internet. With IPv6, every connected device can possess its own globally routable address, eliminating the need for network address sharing.

In reality, most service providers run both technologies in parallel. CGNAT offers an immediate answer to IPv4 exhaustion, while IPv6 deployment progresses gradually across networks. The transition is ongoing but remains slow worldwide due to infrastructure costs, legacy systems, and uneven application support.

A dual-stack architecture has become the most practical approach: subscribers receive both an IPv6 address and an IPv4 address that sits behind CGNAT. Applications capable of using IPv6 communicate directly, bypassing translation layers and reducing demand on CGNAT systems. Yet, because IPv6 compatibility still varies widely across devices, regions, and software, CGNAT continues to serve as an essential bridge between today’s mixed environments and the fully IPv6-driven internet of the future.

There are several options for deploying CGNAT: as dedicated hardware appliances, virtualised software on x86 servers, or even containerised for cloud-native environments. It can also be delivered as a standalone solution or as part of a router or firewall. Choosing the right approach should be based on criteria that are most important for the operator’s business and technical needs.

CGNAT is not the end goal but a bridge. The IPv6 rollout is the only long-term solution, offering nearly unlimited addresses and restoring end-to-end connectivity. That said, adoption remains slow worldwide due to costs, legacy devices, and uneven application support.

In the meantime, operators use dual stack, assigning subscribers both an IPv4 address (often behind CGNAT) and an IPv6 address. This allows IPv6-capable applications to connect directly without translation, reducing load on CGNAT systems.

NFWare CGNAT is a high-performance, fully virtual solution that helps operators manage IPv4 exhaustion while keeping networks scalable and cost-efficient. Built to run on standard x86 servers, it delivers carrier-grade throughput — from tens up to hundreds of gigabits per second per server — with the flexibility to scale capacity step by step as demand grows.

Functionality goes far beyond simple address translation. The platform supports NAT44 and NAT64, advanced logging with Port Block Allocation, high availability with redundancy and failover, and robust routing capabilities. Features like endpoint-independent mapping and filtering, hairpinning, and application awareness ensure smooth operation of VoIP, gaming, and peer-to-peer applications.

With this combination of performance, rich functionality, and simple scalability, NFWare CGNAT provides operators with a practical way to extend IPv4 resources today while preparing for a gradual, future-proof transition to IPv6.