All ISPs receiving /16 prefix blocks from the InterNIC will be responsible for maintaining all IN-ADDR.ARPA domain records for their respective customers. The InterNIC Registry will only be responsible for the maintenance of IN-ADDR.ARPA domain records for those CIDR blocks with prefixes longer than /16 issued directly from the InterNIC. I think you mean shorter, as in: The InterNIC Registry will only be responsible for the maintenance of IN-ADDR.ARPA domain records for those CIDR blocks with prefixes SHORTER than /16 issued directly from the InterNIC. No, I think he means longer. IN-ADDR.ARPA can only be delegated on octet boundaries, so IN-ADDR for /16 and shorter prefixes will be delegated in /16 chunks. IN-ADDR for prefixes longer than /16 must still be maintained by the root, since they cannot be delegated. For example, assume an ISP has been allocated the prefix 204.160/14. Along with this, the InterNIC will delegate the corresponding IN-ADDR domains: 160.204.IN-ADDR.ARPA 161.204.IN-ADDR.ARPA 162.204.IN-ADDR.ARPA 163.204.IN-ADDR.ARPA If the ISP were instead only assigned 204.160.0/17, the associated IN-ADDR range would be: 0.160.204.IN-ADDR.ARPA ... 127.160.204.IN-ADDR.ARPA which is not octet-aligned and therefore cannot be delegated. --Vince