Overview of CGN

This section describes basic concepts and related technologies of CGN.

Telecommunications carriers vigorously deploy mobile Internet, develop broadband users, and proactively promote the triple-play network. However, they must face the reality that IPv4 addresses are exhausted. The Internet Assigned Numbers Authority (IANA) assigned the last global IPv4 addresses on February 3, 2011. Global communications carriers are already facing the lack of IP addresses. At present, there are two methods to resolve the problem:

IPv6 introduction: This method can resolve the address exhaustion problem from the root. However, most content and applications are based on IPv4. Forcible switchover to IPv6 may bring the risk of failing to inherit existing services.
Network Address Translation (NAT): This method continues the use of IPv4 to develop services. Public IPv4 addresses can be reused by deploying private IPv4 addresses on a large scale, which can resolve the IPv4 exhaustion problem for a long term.

Carriers need to consider the research and deployment of both methods.

The NAT method does not require replacement of home gateway devices, greatly reducing carriers' investment costs. If the existing devices can be fully used, carriers' investments can be further saved. This method is most efficient in resolving the IP exhaustion problem.

Carrier Grade NAT (CGN), also called Large Scale NAT (LSN), have improvement regarding the number of concurrent users, performance, and tracing, compared to normal NAT. CGN can be deployed on a large scale by carriers commercially and resolve the IPv4 exhausting problem quickly.

CGN includes various technologies and application scenarios, such as NAT444 and DS-Lite, as shown in Table 1.

**Table 1** CGN technologies
Technology	Description	Remarks
NAT444	NAT can be deployed at both the user gateway and carrier egress for two-level address translation, improving the reusing rate of IPv4 addresses.	-
PCP	The Port Control Protocol (PCP) controls forwarding of incoming packets using an upstream device (such as a NAT444 device or firewall) and also reduces applications' keepalive traffic.	It applies to NAT444 scenarios. The PCP technology, based on P2P, can enable such services as file sharing, voice communication, and video transmission to run very well in NAT444 scenarios.
Static mapping	Mapping between the private IP address pool, public IP address pool, and port range is configured statically. Address translation and authentication are based on the static mapping.	It applies to NAT444 scenarios for quick tracing of user addresses.
DS-Lite	This technology combines both the tunnel and NAT, allowing private IPv4 users to traverse the IPv6 network to access the IPv4 Internet.	-
Port pre-allocation and incremental allocation	These two technologies are enhanced based on NAT. A port range is pre-allocated before NAT for a user, so subsequent services from this user will use this port range for processing.	They apply to NAT444 and DS-Lite scenarios, featuring fast tracing and reduction of logs.
NAT64	This technology enables IPv4 and IPv6 networks to communicate.	-