Well, I understand the concept of Subnet Mask, the thing I don't really get are the "Subnets"

For exemple, with

I could have 8 networks with 62 hosts each. The thing is: I don't get it what does it actually mean.

They all have the Subnet Mask, but they are also different networks. How is this possible?


The subnet mask just encodes the number of bits in the host portion.

/23 = = 9 bits in the host portion

Notice that in, the first 23 bits are set indicating they are part of the network number and the last 9 bits are cleared, indicating they are part of the host portion.

There can be any number of subnets that all have 9 bits in the host portion. They just all have different network numbers.

  • I get that David, but the thing is, for exemple, if you use one of the many tools you find on the internet (like this one: ccna.exampointers.com/sub.php) you see that there are many different "subnets" and every subnet has her own broadcast IP, first usable IP and last usable IP. I wonder: how is this possible if all the subnets have the same submask network? – Con7e Oct 25 '12 at 21:25
  • They have the same subnet mask, but not the same values in the address which is encoded on the numbers indicated in that mask. – Hennes Oct 25 '12 at 21:46
  • I'm not sure what you mean by "submask network". But, again, the subnet mask just encodes the number of bits in the network and host portions of the address. Every single /24 on the Internet has a 24-bit network address, whether it's or Since they all have 24 bits in the network portion, their subnet mask is 24 zero bits followed by 8 one bits or – David Schwartz Oct 26 '12 at 2:02

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