Delegating Reverse-Mapping for Networks with Non-Octet Masks

6.4.1 Problem

You want to delegate responsibility for reverse-mapping a network's IP addresses to a set of name servers, but the network mask or subnet mask doesn't fall on an octet boundary.

6.4.2 Solution

Determine how many reverse-mapping zones correspond to the network. Here's a table to show you how many you'll have.

Size of network

Number of reverse-mapping


128 reverse-mapping zones such as


64 ""


32 ""


16 ""


8 ""


4 ""


2 ""


128 reverse-mapping zones such as


64 ""


32 ""


16 ""


8 ""


4 ""


2 ""


See Section 6.5

For example, say your network is 10/8 and you want to delegate the reverse-mapping zones for the subnet 10.192/10 to your European subsidiary's name servers. That subnetwork corresponds to 64 reverse-mapping zones, from to That's a lot of delegation to add to the zone data file: if each zone has two name servers, that's 128 NS records!

BIND name servers give you a shortcut, though. You can use the $GENERATE control statement to add a group of records that differ only by a number. For example, if your European subsidiary's name servers are called and, you could delegate all 64 reverse-mapping zones to them with just these two $GENERATE control statements:

$GENERATE 192-255 $ NS
$GENERATE 192-255 $ NS

6.4.3 Discussion

Unfortunately, there's no corresponding way to generate the zone statements you'll need on the name servers the zones are delegated to. You'd need a full 64 zone statements in each name server's named.conf file.

6.4.4 See Also

Section 6.5 for handling networks smaller than a /24, and "Subnetting on a Non-Octet Boundary" in Chapter 9 of DNS and BIND.

Getting Started

Zone Data

BIND Name Server Configuration

Electronic Mail

BIND Name Server Operations

Delegation and Registration


Interoperability and Upgrading

Resolvers and Programming

Logging and Troubleshooting


DNS & BIND Cookbook
DNS & BIND Cookbook
ISBN: 0596004109
EAN: 2147483647
Year: 2005
Pages: 220
Authors: Cricket Liu © 2008-2020.
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