I have been reading http://www.centos.org/docs/5/html/Deployment_Guide-en-US/s1-bind-zone.html but the document raised a question. What is the meaning of IN? Is it just a class which is optional or does it mean more than that?

(Yes I read that in named.conf IN is a class statement but I don't know if the same applies to the zone files)

I understand that if I don't define a class "IN" in the named.conf zone statement then I don't have to use it either in the zone files like for example:

With class statement


zone "example.com" IN {   

file "example.com.zone";   



server1 IN  A        
server2 IN  A

Without class statement


zone "example.com" {   

file "example.com.zone";   



server1 A        
server2 A 

In BIND's zone file as well as in named.conf, IN is a class.

You can omit it in any of the files or in both, in any case if class is not explicitly specified, the default "IN" is used.

Regarding the meaning of "IN" - RFC 1035 section 3.2.4:

The following CLASS mnemonics and values are defined:

IN              1 the Internet
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  • +1 for pointing to the definition source. "Back in the day" there were other classes besides IN, but they're basically all dead. – Jeff Ferland Feb 21 '12 at 18:01

Always thought it means Internet Number. According to Wikipedia is actually just Internet:

The CLASS of a record is set to IN (for Internet) for common DNS records involving
Internet hostnames, servers, or IP addresses.
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