How to Calculate Network and Broadcast Address

1. For a classful network total bits is 8. So Total bits = Tb = 8.
   • Sub-net mask can be 0, 128, 192, 224, 240, 248, 252, 254 and 255.

     

   • Below table gives you the "Number of bits used for subnetting"(n) to their corresponding subnet mask.

     
   • For subnet mask 255 is default, so it'll not consider for subnet masking.
   • For example:
     Let, IP address = 210.1.1.100 and Sub-net mask = 255.255.255.224
     
     Total bits = Tb = 8 Number of bits used for subnetting = n = 3 (as subnet mask = 224 and its corresponding "No. of bits used for Subnetting" is 3 from above table)

     

2. 

   2

   From the previous step, you got the "Number of bits used for subnetting"(n) and you know the "Tb", then you can get "Number of bits left for host"(m) = Tb - n as total bits is the summation of number of bits used for subnetting and number bits left for host i.e. Tb = m+n.
   • Number of bits left for host = m = Tb - n = 8 - 3 = 5

     

3. 

   3

   Now you have to calculate "Number of subnets" = 2n and "Value of last bit used for subnet masking"(Δ) = 2m. Number of host per subnet = 2m - 2.
   • Number of subnets = 2n = 23 = 8
     
     Value of last bit used for subnet masking = Δ = 2m = 25 = 32

     

4. 

   4

   Now you can find previously calculated number of subnets by separating subnets each having "Value of last bit used for subnet masking" or Δ addresses.
   • The 8 subnets (as calculated in previous step) are shown above.
   • Each of them has 32 addresses.

5. 

   5

   Now you have to find that your IP address is in which subnet, that subnet's first address is network address and last address is broadcast address.
   • Here the taken IP address is 210.1.1.100 . 210.1.1.100 comes in 210.1.1.96 - 210.1.1.127 subnet (see the previous step table). So 210.1.1.96 is network address and 210.1.1.127 is broadcast address for the taken IP address i.e. 210.1.1.100 .

     

 

Method 2

For CIDR

1. 1

   In CIDR, you have a IP address followed by bit-length prefix separated by slash(/). Now you have to convert bit-length prefix to quad-dotted decimal representation. To do this follow below steps.
   1. Write the the bit-prefix in below format.

      

      • If it's 27, then write it as 8 + 8 + 8 + 3 .
      • If it's 12, then write it as 8 + 4 + 0 + 0 .
      • Default is 32, which is 8 + 8 + 8 + 8.
   2. Convert the corresponding bit according to the below table and represent in quad-dotted decimal format.

      

   3. Let an IP address be 170.1.0.0/26 . Using above table, you can write:
       

      										26	=	8	+	8	+	8	+	2
      												255	.	255	.	255	.	192

      Now the IP address is 170.1.0.0 and subnet mask in quad-dotted decimal format is 255.255.255.192 .

      

2. 2

   Total bits = Tb = 8.
   • Sub-net mask can be 0, 128, 192, 224, 240, 248, 252, 254 and 255.
   • Below table gives you the "Number of bits used for subnetting"(n) to their corresponding subnet mask.

     

     
      
   • For subnet mask 255 is default, so it'll not consider for subnet masking.
   • From the previous step, you got IP address = 170.1.0.0 and Sub-net mask = 255.255.255.192
     
     Total bits = Tb = 8 Number of bits used for subnetting = n = 2 (as subnet mask = 192 and its corresponding "No. of bits used for Subnetting" is 2 from above table)

     

3. 

   3

   From the previous step, you got the "Number of bits used for subnetting"(n) and you know the "Tb", then you can get "Number of bits left for host"(m) = Tb - n as total bits is the summation of number of bits used for subnetting and number bits left for host i.e. Tb = m+n.
   • Number of bits left for host = m = Tb - n = 8 - 2 = 6

4. 

   4

   Now you have to calculate "Number of subnets" = 2n and "Value of last bit used for subnet masking"(Δ) = 2m. Number of host per subnet = 2m - 2.
   • Number of subnets = 2n = 22 = 4
     
     Value of last bit used for subnet masking = Δ = 2m = 26 = 64

     

5. 5

   Now you can find previously calculated number of subnets by separating subnets each having "Value of last bit used for subnet masking" or Δ addresses.
   • The 4 subnets (as calculated in previous step) are

     

   • Each of them has 64 addresses.

     

6. 

   6

   Now you have to find that your IP address is in which subnet, that subnet's first address is network address and last address is broadcast address.
   • Here the taken IP address is 170.1.0.0 . 170.1.0.0 comes in 170.1.0.0 - 170.1.0.63 subnet (see the previous step table). So 170.1.0.0 is network address and 170.1.0.63 is broadcast address for the taken IP address i.e. 170.1.0.0 .