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Benefits of Carrier Grade NAT
Benefits of Carrier Grade NAT
CGN offers these benefits:
• Enables service providers to execute orderly transitions to IPv6 through mixed IPv4 and IPv6 networks.
• Provides address family translation but not limited to just translation within one address family.
• Delivers a comprehensive solution suite for IP address management and IPv6 transition.
IPv4 Address Shortage
A fixed-size resource such as the 32-bit public IPv4 address space will run out in a few years. Therefore, the
IPv4 address shortage presents a significant and major challenge to all service providers who depend on large
blocks of public or private IPv4 addresses for provisioning and managing their customers.
Service providers cannot easily allocate sufficient public IPv4 address space to support new customers that
need to access the public IPv4 Internet.
NAT and NAPT Overview
A Network Address Translation (NAT) box is positioned between private and public IP networks that are
addressed with non-global private addresses and a public IP addresses respectively. A NAT performs the task
of mapping one or many private (or internal) IP addresses into one public IP address by employing both
network address and port translation (NAPT) techniques. The mappings, otherwise referred to as bindings,
are typically created when a private IPv4 host located behind the NAT initiates a connection (for example,
TCP SYN) with a public IPv4 host. The NAT intercepts the packet to perform these functions:
• Rewrites the private IP host source address and port values with its own IP source address and port
• Stores the private-to-public binding information in a table and sends the packet. When the public IP host
Traditionally, NAT boxes are deployed in the residential home gateway (HGW) to translate multiple private
IP addresses. The NAT boxes are configured on multiple devices inside the home to a single public IP address,
which are configured and provisioned on the HGW by the service provider. In enterprise scenarios, you can
use the NAT functions combined with the firewall to offer security protection for corporate resources and
allow for provider-independent IPv4 addresses. NATs have made it easier for private IP home networks to
flourish independently from service provider IP address provisioning. Enterprises can permanently employ
private IP addressing for Intranet connectivity while relying on a few NAT boxes, and public IPv4 addresses
for external public Internet connectivity. NAT boxes in conjunction with classic methods such as Classless
Inter-Domain Routing (CIDR) have slowed public IPv4 address consumption.
Network Address and Port Mapping
Network address and port mapping can be reused to map new sessions to external endpoints after establishing
a first mapping between an internal address and port to an external address. These NAT mapping definitions
are defined from RFC 4787:
Cisco IOS XR Carrier Grade NAT Configuration Guide for the Cisco CRS Router, Release 5.2.x
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values
returns a packet, it is addressed to the NAT. The stored binding information is used to replace the IP
destination address and port values with the private IP host address and port values.
Implementing Carrier Grade NAT on Cisco IOS XR Software
OL-32659-01
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