BIND/DNS Zone meaning of "IN"

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.com.zone

server1 IN  A   10.0.1.5        
server2 IN  A   10.0.1.6

Without class statement

named.conf

zone "example.com" {   

file "example.com.zone";   

};

example.com.zone

server1 A   10.0.1.5        
server2 A   10.0.1.6 

Solution 1:

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
...

Solution 2:

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.