Format you have specified : <ip-address>/<network-segment-bit-length>
Step 1. write the network mask in binary - first 10 digits are 1 and the rest are 0 (10 being the <network-segment-bit-length>)
11111111110000000000000000000000
Step 2. Break the sequence in to 8 bit segments.
11111111.11000000.00000000.00000000
Step 3. Convert each segment in the bit string to decimal separately to get 255.192.0.0
11111111.11000000.00000000.00000000
255. 192. 0. 0
Additional: the meaning of sub net.
Step 1. Convert the ip address to bits (each segment separately).
10. 10. 10. 0
00001010.00001010.00001010.00000000
Step 2. Isolate first 10 bits and the rest in to two groups
A: 00001010.00------.--------.--------
B: --------.--001010.00001010.00000000
Part A is the network segment. Part B is the host segment.
Step 3. Fill in the blanks for part A with 0's and convert to decimals (each segment separately) to calculate the first possible address for the sub-net
00001010.00000000.00000000.00000000
10. 0. 0. 0
Step 4. Fill in the blanks for part A with 1's and convert to decimals (each segment separately) to calculate the last possible address for the sub-net
00001010.00111111.11111111.11111111
10. 63. 255. 255
Conclusion: Any IP address in the range of 10.0.0.0 to 10.63.255.255 falls in the specified sub-net.