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Explain me this subnetting question?

gouki2005gouki2005 Member Posts: 197
in CCNA & CCENT
How many subnets and hosts per subnet can you get from the network 10.0.0.0/20?

Answer: 4096 subnets and 4094 hosts

ok i understan the host part is /20 so 32-20 = 12 and 2^12 = 4096 host but i dont understand the subnets part if this one is a class 9 = 11111111.11111111.11110000.00000000 so = 4 for subnets

i get it now the mask is a class b but the address is a class A so 8+4 = 12 subnets right

so the mandatory number is the address right i must the calculation depends of the address no the mask right???
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    MosGuyMosGuy Member Posts: 195
    The answer to the original question would be:

    10.0.0.0 /20
    255.0.0.0

    11111111.11111111.11110000.00000000

    Since the default mask is: 255.0.0.0, the first octet won't change. You add 12 bits starting in the second octet. You can think of it as: 20-8 (8 being the number of network bits in the default mask)

    - 12 network bits: 2^12 = 4,096
    - 12 host bits: 2^12-2 = 4,094
    Subnet mask of 255.255.240.0


    I think you're getting confused seeing the sub-netted mask i.e: 255.255 and thinking class B but don't. Look at what the IP address starts with i.e 10.0.0.0 = Class A. The fact the mask is 255.255.240.0 rather then the default 255.0.0.0, just means the address is sub-netted already. The key is to go by the class default mask i.e: A=255, B=255.255, C=255.255.255


    A similar question for a class B address with a /20 would give:

    Original mask: 255.255.0.0

    11111111.11111111.11110000.00000000

    4 network bits: 2^4= 16 subnets
    12 host bits: 2^12-2=4.094
    Subnet mask of: 255.255.240.0


    This likely isn't the best explanation, but I hope it may help a little.
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