How to find subnet mask

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Subnet mask
Subnets
If we have an IP of 211.17.180.0/24
  1. What does the subnet mask means. How it can be calculated?
  2. What is /24 means?
  3. How total subnets can be calculated?
  4. How total address can be calculated?
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  • Brijesh
    1. The Subnet mask is generally used to identify network address of an IP address by perfoming a bitwise AND operation on the netmask. A Subnet mask is a 32-bit number that masks an IP address, and divides the IP address into network address and host address.

    In-order to calculate the subnet mask , use the below URL,

    http://www.aboutmyip.com/AboutMyXApp/SubnetCalculator.jsp

    2. /24 means no of bits masked from the host bit of the IP address range. 
    Its used to denote the subnet mask of the IP address. 

    Subnet mask is also a 32 bit address same as IP address. 

    As the above example is already a Class C IP address and as no bits have been borrowed for further subnetting therefore its denoted by /24 meaning it would have a default subnet mask for Class C IP address which would be 255.255.255.0 

    For Eg: If it had been /25, then the subnet mask for the above IP address range would have changed to 255.255.255.128 as one 1 bit from host range would have been borrowed. 
     

    3. Total number of subnets are calculated for one subnet mask.

    For example, take this subnet mask: 255.255.255.224 and IP address as 192.168.5.139,  ICND1 boook shows that there are 6 subnet for the given subnet mask, ranging from 192.168.5.32 to 192.168.5.192 , how 6 ? what to consider in ?

    The default mask for the class C address of 192.x.x.x is 255.255.255.0
     
    The mask in the example is 255.255.255.224 (using 3 extra bits for subnetting, above and beyond the default).
     
    2^3 is 8 combinations:
     
    000
    001
    010
    011
    100
    101
    110
    111
     
    In the old days, we couldn't use the all 000 or all 111 combination for subnetting, so the formula was 2^extra bits used for custom subnetting, or 2^3 in our example, -2 (for the 2 that used to not be able to be used), which would leave 6.
     
    Today, current IOS has the ability to use the 000 and 111 option with an option called subnet zero, and so on current IOS there would be 8 possible subnets, not just 6.
     
    The possible subnets would be 192.168.5.0/27
    the next subnet would jump by 32, to 192.168.5.32/27,
    and they would continue to increase by 32 until the subnet of 192.168.5.224/27 was reached, for a total of 8 subnets.
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