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Netmask Ordering

February 20, 2013

I’ve been playing with my test environment tonight and came across a feature I didn’t know about and was causing me some issues, so here is what I found.

Netmask ordering if enabled in DNS advanced options, by default returns records that match based on the 3rd octet (eg. 10.10.x.2 x=3rd octet).

So of your doing a query for a list of DCs for instance and there are a number of subjects in your site, the order on which they are returned will be determined by the client machines 3rd octet. For instance if your DCs are 10.10.1.1, 10.10.2.1 and 10.10.3.1 and your client machine is 10.10.2.50. When the client does a DNS lookup it will be returned with the 10.10.2.1 DC first in the list. If your client IP was 10.10.5.50 then the list order would be returned in round robin order.

If you want to change it to class B, then run the following command:
Dnscmd /Config /LocalNetPriorityNetMask 0x0000FFFF

A useful resource to understand this is:
http://rdd.me/gxhieky0

Mark

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