Unnumbered IP is a feature that enables you to configure a point-to-point interface to borrow an IP address from another interface. Unnumbered IP enables IP processing on a point-to-point interface without assigning an explicit IP address to the interface.

Although unnumbered IP is supported on all point-to-point interfaces, the main use of the feature is to enable dynamic routing on the Dialer interface. The Dialer interface is used for the modem dial-backup feature. Refer to Modem dial backup. Modem dial-backup is a feature that sets up a backup dialing destination for a Branch Gateway. Modem dial-backup requires unnumbered IP to be configured on the Dialer interface of the Branch Gateway and at both the default and the backup dialing destinations.