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This question is bit novice. But, just wanted to clear any doubts as a starter for this topic. I have read about Classes of address like Class A, B, C. As I observe the Default Subnet mask for each of these classes, I see that, starting from Class A, gradually, network mast/prefix length increases.

Default subnet masks: Class A: 255.0.0.0 (prefix length : 8) Class B: 255.255.0.0 (prefix length : 16) Class C: 255.255.255.0 (prefix length : 24)

This p appears to me like, if a network administrator uses Class C address, then, he will be limited to have maximum of 254 hosts (excluding 0 and 255), whereas, if he chooses Class A address, then, he can have maximum number of hosts to give IP addresses to. Is this perception correct? So, Class A is always better, even if some IP addresses are wasted in the beginning, it can always grow in future? So, my question is, on which condition or constraint, a particular Class of address will be picked by the network admin? Is he free to choose any class of address? Also, I read that, starting octet of a particular class of address needs to fall into a range. For example, for Class C, it has to be 192-223 (inclusive). That, means, if network admin chooses, Class C address with default subnet mast (255.255.255.0), then one of the host can have an IP address like 192.0.0.1 or 223.0.0.1? Is it legal to have all 0's in the network part of the address also?

Also, another query is, does the subnet mast denote maximum number of hosts possible in the network and can be reduced by subnetting? For example, as per my understanding, netmask can be further changed for subnetting, like if a Class C default netmask 255.255.255.0 is converted to 255.255.255.192, network admin can break the original network into 4 subnets?

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    I have read about Classes of address like Class A, B, C. Throw that book out.
    – Ron Trunk
    Oct 6 at 12:11
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    See this answer.
    – Ron Maupin
    Oct 6 at 12:36
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Network classes are long dead, obsoleted in 1993 by CIDR, before the Internet went commercial. You should dump any book on IP from before that year. Classful networking may be interesting for historical reasons but it's completely irrelevant in practical use today.

For a private network, just use any subnet(s) from the RFC 1918 ranges 192.168.0.0/16, 172.16.0.0/12 and 10.0.0.0/8. There's no one-size-fits-all, but for most purposes, /24 subnets are the most reasonable.

For subnetting in general, check out these excellent answers.

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  • 10.x.x.x/23 could be implemented then? 172.16.x.x/14?
    – hakkican
    Oct 6 at 13:22
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    Yes, those subnets are fine to use. You can super- or subnet freely as long as you stay within the RFC 1918 ranges. (You can also choose to ignore RFC 1918 but would forfeit any chance for trouble-free Internet connectivity - so don't do that.)
    – Zac67
    Oct 6 at 16:04
  • online subnet calculators still force to use class a,b,c to calculate.So they are wrong and lead people to think network classes still exist.
    – hakkican
    Oct 7 at 5:56
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    That's likely because that crap is still taught. With my trainees I used to teach them CIDR first and then mention classful eventually. Forget those calculators, it's easy to do yourself if you've read the linked Q&A.
    – Zac67
    Oct 7 at 6:29

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