The CDA is a 6 byte binary number used to identify a node frame in a node pot, a node pot itself, or a page. The CDA is purely a kernel construct. A CDA can be transformed into a physical disk address with a little arithmetic and a small set of tables of ranges. When a disk is mounted the kernel reads a table of ranges from the disk that announce which CDAs are on this disk. Each range is some substantial portion of the disk. A range is expected to live on more than one disk for purposes of redundancy. Extensive cross checks are in place to ensure that at least one copy of a range is accessible. When a disk is introduced with obsolete ranges a process is begun to repair that range.