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Asked: 2026-05-21T21:31:53+00:00

I need help to discover how to identify how many host addresses are available

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I need help to discover how to identify how many host addresses are available on the mobile network using this subnet mask 255.128.0.0.

I know the answer is 2^23-2 = 8388608 (8388606 – less subnet address and broadcast address)

But why is it this answer. The mobile network is a class A address that has 2^24-2 host addresses. So would the number of hosts on the subnet mask always be 1 less, is this why it is 23?

Or is this because as there is 128 shown after 255 and this is 1000 0000 that one bit is occupied and is less than 24?

I will be very grateful for an answer as I have looked everywhere. Thanks

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1 Answer

  1. Editorial Team

    Subnet masks and network classes are related but distinct.

    You say that the base network is a class A network. That indicates to me that it is some network with an address range of n.0.0.0 to n.255.255.255, where n <= 127. (See https://en.wikipedia.org/wiki/Class_A_network#Introduction_of_address_classes.) This class A network itself has an effective subnet mask of 255.0.0.0, with 24 bits of freedom.

    The subnet mask of 255.128.0.0 represents an additional narrowing of the wider class A network. Apparently the class A network has been partitioned in two by reserving one extra bit. Normally it would be further allocated into fairly narrow subnets.

    Because the subnet mask you provide fixes an additional bit beyond the class A mask, it has only 23 bits of freedom. This is why the exponent in your calculation is 23 rather than 24.

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