CG-NAT (Carrier-Grade Network Address Translation) is a large-scale network address translation technology used by Internet service providers to optimize and conserve IPv4 addresses. Unlike traditional NAT, which is typically implemented in home routers, CG-NAT operates at the carrier level, allowing thousands of subscribers to access the Internet through a limited pool of public IP addresses.
CG-NAT has become widely adopted due to IPv4 address exhaustion and the gradual transition to IPv6.
Core Objectives of CG-NAT
- conserving scarce IPv4 addresses;
- enabling scalability of provider networks;
- ensuring compatibility between subscribers’ private networks and the global Internet.
Applications
CG-NAT is used by ISPs and mobile operators in:
- fixed-line networks (FTTB, GPON, ADSL);
- mobile LTE/5G networks;
- large enterprise networks with numerous users.
Advantages
- IPv4 conservation – allows thousands of clients to be served with a minimal number of public addresses.
- Compatibility – ensures Internet access without modifying customer equipment.
- Transitional solution – serves as an interim measure until full IPv6 adoption.
Limitations and Drawbacks
- No direct access – subscribers cannot host inbound connections (e.g., servers, CCTV, online games).
- Diagnostic complexity – sharing a single IP among many users makes it harder to trace malicious activity.
- Port limitations – providers must allocate port ranges, which can cause issues for some applications.
- Increased latency and load – the extra translation layer adds delays and requires more processing resources.
Example
A mobile operator serves 1 million customers but has only 50,000 public IPv4 addresses. With CG-NAT, each customer is assigned a private (“gray”) IP, while Internet sessions are distributed across the available public addresses using port allocation. This allows the operator to provide Internet access to all subscribers without assigning a unique public IP to each one.