Thursday, 30 July 2009

Classless IN-ADDR.ARPA delegation

In some cases you may be allocated a block of IP addresses smaller than a /24. This is referred to as a Classless Delegation and is explained in technical detail in RFC 2317.

For example, if your ISP allocated the block (the 32 IP addresses from to, the reverse DNS for this zone would be served out of a zone named

Once the zone is created, you'll be able to create PTR entries within this zone for each individual IP address. As an example, to map the IP address to, you would enter:

Host: 226
TTL: 86400
Type: PTR

Once you have setup the Custom DNS zone for this block, you need to have the block delegated to us. Contact your ISP and have them create NS records for the zone:   86400  IN      NS 86400 IN NS 86400 IN NS 86400 IN NS 86400 IN NS

Your ISP will ALSO need to create individual CNAME records for each IP address they have delegated to you. These CNAME records map the IP addresses into the new name space that has been defined via the zone.   86400  IN     CNAME 86400 IN CNAME 86400 IN CNAME

Until those changes are made, DNS queries for those IP addresses won't come to us, so it is not enough to simply create the zone. You MUST get it delegated to us and have the CNAME entries created in order for the reverse DNS to function.

No comments: