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I am currently learning about subnetting and my tutor has asked a question:

  • Base address: 192.168.0.0, mask: 255.255.255.0
  • Steal 4 bits
  • What is the last address of the first usable subnet?

I know how to find the first, last, broadcast address but what is meant with first usable subnet?

Gijs de Blauw
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  • Every subnet is usable. There are some bad (very bad) classes that teach that the first and last subnets are unusable, which was once a myth that was explicitly dispelled by an RFC. If that is what is being taught, it is incorrect. See [this two-part answer](https://networkengineering.stackexchange.com/a/53994/8499) that explains all about IPv4 addressing and subnetting. – Ron Maupin Mar 19 '19 at 23:43

1 Answers1

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I believe the person asking about the last usable IP in the Class C subnet they provided. If you break down that subnet into binary you would get:

11111111.11111111.11111111.00000000
--------------------------> /24 as there are 24 1's

Effectively there are 65536 available subnets in a /24 and 256 hosts per subnet. This is a simple question due to it being a /24 meaning the last octect is the only octect you are going to be using. Like someone mentioned in the comments, every subnet is usable but the "first" usable subnet would be 192.168.0.0/24 which would give you the usable range between .1->.254 leaving .0 as the network identifier and .255 for the broadcast.

Mike Hayes
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  • Network classes were killed in 1993 (please let them rest in peace) by RFCs 1517, 1518, and 1519, which defined CIDR (_Classless_ Inter-Domain Routing). Modern networking does not use network classes (possibly since before you were born). If a class is teaching network classes and that the first and last subnet are unusable, you should demand your money back because it is not based in reality. – Ron Maupin Mar 20 '19 at 02:55
  • Not saying the first / last subnet is not usable, saying the first / last host in each subnet is not usable (network / broadcast). I think his tutor was asking what the first subnet available to the person is which I would assume 192.168.0.0/24 -edit- I misread your message. I get what you're saying. fwiw, classes are still asked on the CCNA – Mike Hayes Mar 20 '19 at 02:57
  • The problem with your answer is that it assumes there are network classes, which were deprecated (1993) two years before the Internet went commercial (1995). _Nothing_ in modern networking uses network classes, and the Internet could not exist if there were network classes. – Ron Maupin Mar 20 '19 at 03:00
  • All you really need to do to understand that network classes are dead is to look at the answers and comments on [networkengineering.com]. A simple search turns up all these hits: https://networkengineering.stackexchange.com/search?q=network+class – Ron Maupin Mar 20 '19 at 03:15