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

For exemple, with 143.143.0.1/23

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 255.255.254.0, but they are also different networks. How is this possible?

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

/23 = 255.255.254.0 = 9 bits in the host portion

Notice that in 255.255.254.0, 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.

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  • 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, 2012 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, 2012 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 192.168.31.0/24 or 216.152.51.0/24. Since they all have 24 bits in the network portion, their subnet mask is 24 zero bits followed by 8 one bits or 255.255.255.0.
    – David Schwartz
    Oct 26, 2012 at 2:02

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