Networking 101 : IP address classes and VLSM - Variable-Length Subnet Mask -

Below are the IP classes:
  • Class A : Ranges from "1.n.n.n" to "126.n.n.n". With class A we get 127 networks, each with up to sixteen million hosts. 
  • The first octet - 1 to 126 - is used for the network address, and the rest - n.n.n - is used for the host IP address.
  • Class B : goes from "128.n.n.n" to "191.n.n.n". With the class B address, we can have sixteen thousands networks with sixty five thousands each.
  • Class C : goes from "192.n.n.n" to "223.n.n.n". With the class C address, we can have two million networks with two hundreds and fifty five million hosts each.
  • Class D : goes from "224.n.n.n" to "239.n.n.n", and it is used for multicast addressing.
Below is an example of how the network and the host parts of the IP address - affected by the network mask:

The above part represents the value of each bit if activated - equals to 1 -.

Variable-Length Subnet Mask extends the standard subnetting to give us more flexibility in terms of addressing.

Below is a simple example:

To know the network part, we take into account the bits
that have a value of “1” for both the IP address and the subnet mask - in red - that could be expressed also as "/26".


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