Carrier-Grade Network Address Translation (CGNAT), also known as Large Scale NAT (LSN), is a technology for managing the growing demand for IPv4 addresses in large Internet Service Provider (ISP) networks. CGNAT makes it possible to hide multiple customers behind a common public IPv4 address by converting private IPv4 addresses into public IPv4 addresses. This article provides an overview of CGNAT, its functionality, benefits and challenges.

Background and necessity #

The need for CGNAT arises from the limited supply of available public IPv4 addresses. With the increasing number of Internet-enabled devices and the expansion of the Internet, the available IPv4 address space was quickly exhausted. This meant that ISPs had to use their IPv4 addresses more efficiently. CGNAT makes it possible to hide thousands of customers behind a single public IPv4 address, alleviating the scarcity of available addresses.

How CGNAT works #

CGNAT works in a similar way to conventional Network Address Translation (NAT), but on a much larger and more scalable level. The basic principles are as follows:

1. Private IP addresses: Customers in an ISP network receive private IPv4 addresses that cannot be routed directly on the public Internet.
2. Mappingtables: The CGNAT router creates a mapping table that tracks the assignment of private to public IP addresses. This table contains entries for each active connection.
3. Port translation: One of CGNAT’s main techniques is to perform port translations. This means that several private IP addresses can share the same public IP address and port range.
4. Packet forwarding: When data packets from a customer reach the public IPv4 address and port number, the CGNAT router performs the corresponding translations and forwards the packet to the correct customer.

Advantages of CGNAT #

1. IPv4 resource optimization: CGNAT allows ISPs to use their limited IPv4 addresses more efficiently by hiding multiple customers behind one public address.
2. Cost reduction: Extending the lifetime of IPv4 addresses through CGNAT can delay the need to migrate to IPv6 and the associated costs.
3. Security: CGNAT can provide some security as it hides customers behind a shared public port range, making potential attacks more difficult.

Challenges and concerns #

1. Restrictions for peer-to-peer applications: CGNAT can impair peer-to-peer applications as it makes it difficult to reach end devices directly.
2. Complicated IPv6 transition: The long-term solution to the shortage of IPv4 addresses is the transition to IPv6. CGNAT can ease the pressure on the transition to IPv6, which is necessary in the long term.
3. Problems with some applications: Some applications that rely on fixed IP addresses or specific ports may be affected by CGNAT.