1. A method in a mobile router configured for establishing communications within an ad hoc network, the method comprising: attaching to an attachment router based on an advertisement message from the attachment router that specifies an attachment prefix, and according to a protocol requiring establishment in the ad hoc network of a tree topology having a single clusterhead, and based on selecting a default attachment address within an address space of the attachment prefix; and outputting to the attachment router a neighbor advertisement message specifying that a prescribed address prefix used by the mobile router is reachable via the default attachment address, based on the mobile router having attached to the attachment router, the prescribed address prefix distinct from the attachment prefix.