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Some sources say that class A IP addresses start at 1.0.0.0, and some say 0.0.0.0. I have also heard that 0.0.0.0 is a special kind of IP address. So is 0.0.0.0 a class A IP address or not?

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closed as off topic by ThatGraemeGuy, Hyppy, Shane Madden, Ward, Scott Pack May 20 '11 at 13:25

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There are so such thing as Classes anymore; CIDR replaced them 17 years ago. Your teacher should be fired for even mentioning them outside of a history class. –  Chris S May 19 '11 at 12:50
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4 Answers

http://tools.ietf.org/html/rfc3330 Explains it.

0.0.0.0/8 - Addresses in this block refer to source hosts on "this" network. Address 0.0.0.0/32 may be used as a source address for this host on this network; other addresses within 0.0.0.0/8 may be used to refer to specified hosts on this network [RFC1700, page 4].

So it is Class A but a special adress also.

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Network classes are defined by their leading bits. Class A's leading bit is 0, which means the first octet can be 0-127. 0.0.0.0 is indeed a class A address.

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It falls within the class A range, but it is not a useable address (as an IP). It is special, in that it generally is used to mean "any network" or the default route address. It is used for routing.

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0.0.0.0 is a special address, it is even network (0.0.0.0/32 and so on),that acts like loopback 127.0.0.1 (that is only for local machine), but is accessible for anyone outside. It acts like a default route.

A number of network addresses are reserved for special purposes. 0.0.0.0 and 127.0.0.0 are two such addresses. The first is called the default route, and the latter is the loopback address. The default route has to do with the way the IP routes datagrams.

linux tcp stack

They arent A B C D or even E.

0.0.0.0 is the default route

windows tcpip

But there is the question is what class and there is answers, choose class A.

The address 255.255.255.255 is used as broadcast addresses and 0.0.0.0 as a default route address, meaning any network.

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