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How to configure NAT on a cable modem

Core Issue

Network Address Translation (NAT) replaces IP addresses within a packet with different IP addresses. NAT is useful for conserving IP addresses and connecting a private network using unregistered addresses to a public network like the Internet.

Cable modems like the uBR900 series devices are used to provide Internet access to users through a cable network by connecting to a Cable Modem Termination System (CMTS) device. Cable modems also connect to the Cisco Customer Premises Equipment (CPE) like PCs over the LAN interface. Cable modems can operate in bridging or routing mode. NAT is used on the cable modem when the CPE devices that use an unregistered address need to connect to the Internet. This is done by using a valid address provided by the service provider. Port Address Translation (PAT), a form of dynamic NAT, is used to bind the single valid address to the various unregistered addresses. To identify the uniqueness of a session with the source port number, PAT enables the NAT router, allowing many hosts to share the same address simultaneously.


When configuring PAT on the cable modem, the unregistered addresses that belong to the CPE devices on the LAN interface are translated to the IP address, which is assigned to the cable interface provided by the service provider through DHCP. The DHCP server can run either on the CMTS itself or on another device that can be accessed by the CMTS.

To resolve this issue, perform these steps:

  1. Before configuring NAT, enable routing on the cable modem by issuing the no cable-modem compliant bridge command under the cable interface.

    Remove any bridge groups under the cable and LAN interfaces by issuing the no bridge-group commands.

    These two commands disable the bridging functionality.

  2. To route IP traffic, issue the ip routing command in global configuration mode.
  3. Configure an IP address under the LAN interface by issuing the ip address command.
  4. To permit the addresses of the CPE devices that need to be translated, define Access Control Lists (ACLs) by issuing the access-list command.
  5. Configure PAT by binding the list of hosts translated to the cable interface by issuing the ip nat inside source list name> interface interface_name> overload command in global configuration mode, where interface_name stands for the cable interface.
  6. Issue the ip nat outside command under the cable interface.
  7. Issue the ip nat inside command under the LAN interface.

For a sample configuration, refer to Configuring NAT on Cable Modem.

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Last update:
‎06-22-2009 05:34 PM
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