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question about subnet mask

sdeyoungsdeyoung Member Posts: 109 ■■■□□□□□□□
in Network+
I know that 255.0.0.0 is class A, 255.255.0.0 class B and 255.255.255.0 is class C but what does it mean when say your router is configured to 255.255.255.128? The subnet mask of my ISP is 255.255.255.128 and my router is set the same.. Just curious..
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    mwgoodmwgood Member Posts: 293
    Each octet has 8 bits which are either 1 or 0. A subnet mask of 255.255.255.128 looks like this:

    11111111.11111111.11111111.10000000

    The decimal value of the last octet = 128.

    The 1's represent the (sub)network address, while the 0's represent the host portion (with the last one being the broadcast address).

    This could also be represented using the CIDR notation of /25 - since the (sub) network address contains 25 bits.
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