I hope this question fits this section of the forum.

I've been spending some time learning subnetting and practicing the questions on http://www.subnettingquestions.com/ . I saw this question and was confused.


Question: How many subnets and hosts per subnet can you get from the network 172.30.0.0 255.255.255.192?

Answer: 1024 subnets and 62 hosts

Seeing 62 hosts is easy, you just figure out the range of that network...
Network: 172.30.0.0
First usable: 172.30.0.1
Last usable: 172.30.0.62
Broadcast: 172.30.0.63

But how in the heck are they getting 1024 subnets from that small range? I can see getting a few /29's out of that, but nothing near 1024.

I'm either reading the question wrong or there's an error with the answer.... I'm betting on the former.

Any help is very much appreciated.

Thank you.

Hi there,

I was confused, too, because classful subnetting has been obsolete for about 20 years now. You're reading the wrong sources. It's like you'd study electronics today and put your main focus on vacuum tubes. Silly. Obsolete.

The question doesn't make sense unless you're considering the historical network classes. Obviously they imply a class B network here as a basis. Historical Class B is netmask 255.255.0.0 or /16 in modern CIDR.

Let's get into that assuming we're really talking about stone-age basics.

If you have to stay inside the Class B network range, you must not touch the mask bits I labeled with "C" (for "Class"). That leaves you 10 bits for the network address, and 6 for the host address. 10 bits make 1024 combinations, 6 bits make 64 combinations. Minus 2 for network and broadcast address leaves 62 possible host addresses in each subnet.

In today's CIDR, there are no classes any more. There are just network and host address bits, the "C" bits in the above example would all be "N".
What remained, however, were the reserved private, non-routed address ranges, like for example 192.168, which is probably the most famous one, and is typically set up in /24 subnets (netmask 255.255.255.0). That's still in compliance with the historical Class C networks. But this is completely up to the system administrator; he might just as well use /20 networks (255.255.240.0), or /27 (255.255.255.224) if that is appropriate for his particular network.

[X] Doc CPU

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What do the calculators say you should get?