IP Unnumbered

IP unnumbered can help you solve the problem of discontiguous networks in your network. Providing the two networks are in the same IP class you can use IP unnumbered.

IP unnumbered essentially uses an IP address from another interface, which will be in the same network rather than a separate serial link network.

This is achieved using the IP unnumbered Ethernet x command within the config-if mode of Cisco's IOS.

IP unnumbered is essentially a "hacked" solution to achieve a contigous network in order that you may use auto summarising within your network. As such, beware that it does come with it's fair share of problems.

Problems with IP unnumbered:

  • You won't be able to ping a serial interface when using IP unnumbered, since they don't have an IP address associated with them.
  • IP security options are not supported over these links.
  • SMDS and X.25 protocols do not support IP unnumbered.

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