BIND/DNS Zone meaning of "IN"

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http://serverfault.com – 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 named.conf zone "example.com" IN { file "example.com.zone"; }; example.co (HowTos)