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This topic contains 1 reply, has 2 voices, and was last updated by  cliffwilliams 1 year, 6 months ago.

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  • #106595

    kalpataru podder
    Participant

    HOW the subnet mask of class b is 255.240.0.0,and what is /12,/8?

    #106766

    cliffwilliams
    Participant

    Firstly the subnet mask 255.240.0.0 will never be found in relation to APIPA addresses. Howver this video discusses both APIPA addresses and private IP addresses.

    The address space 172.16.0.0 – 172.31.255.255 has been allocated for the class B range of private IP addresses. In addition to indicating an address range by the starting and ending addresses you can indicate it by the starting address and a subnet mask. Doing this for the class B range of private addresses gives you the IP address 172.16.0.0 combined with subnet mask 255.240.0.0 Hence the subnet mask 255.240.0.0 indicates a range of 1,048,576 addresses (but note the first represents address for that entire network and the last is the broadcast address for that network).

    The use of the 255.240.0.0 subnet mask to identify the entire class B private address range should not be confused the subnet mask 255.255.0.0 which is the default subnet mask for class B addresses. The range of class B private addresses actually allows for 16 different class B networks.

    The subnet mask 255.240.0.0 when converted to binary is
    11111111 11110000 00000000 0000000
    if you count the number of 1’s at the beginning you will find that their are 12. Also if you convert the subnet mask 255.0.0.0 to binary you get
    11111111 00000000 00000000 0000000
    and if you count the 1’s at the beginning you will find that there are 8. In fact valid subnet masks will always start with a number of 1’s followed by a number of zero’s. You can therefore describe a network mask by simply counting the number of 1’s at the beginning. This gives rise to an alterantive notation for showing a subnet mask or ip range by using a slash followed by the number of 1’s. Hence a /8 indicates a class A range of 255.0.0.0 and /16 indicates a class B range of 255.255.0.0 The /12 you ask about is equivalent to a subnet mask of 255.240.0.0

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