Seven Subnetting Steps
1) Take any address and bit mask, 172.16.25.12 /29, and distribute the subnet bits into w.x.y.z format, ie. 8.8.8.5
2) Take the last non-8-bit value, mark down the bits, filling the rest of the 8 bits with zeroes, ie. 11111000
3) Binary weight each bit from right to left, until you get to the first non-zero bit: the weight of this bit is the subnet multiple. In this example, it is 8. Subtract this value from 256 to get the value for the subnet mask.
128 64 32 16 8 4 2 1
1 1 1 1 1 0 0 0
Weight = 8. 256 – 8 = 248.
4) You have now determined the subnet mask in dotted decimal form, i.e. 255.255.255.248
5) Now that you know the subnet multiple is 8, list the subnets starting with zero. Some examples near the address of 172.16.25.12:
172.16.25.0
172.16.25.8
172.16.25.16
172.16.25.24
……………..
172.16.25.240
172.16.25.248
6) Separate the subnets and draw circles to illustrate the subnet boundaries.
172.16.25.0-----|
|
172.16.25.7 |
-----------------
172.16.25.8-----|
|
172.16.25.15 |
-----------------
172.16.25.16---|
|
172.16.25.23 |
-----------------
In the bottom of the circle, fill in the last address before you move into the next circle, ie. 172.16.25.7, which is the subnet broadcast address
for subnet 172.16.25.0. Exclude the first address in the circle because that is the subnet number, i.e. 172.16.25. 0. All other addresses are valid node addresses, i.e. 172.16.25.1 to .6.
7) Find the lowest subnet that the original address, 172.16.25.12, can fit into, i.e. 172.16.25.8. You now know which subnet it belongs to and its broadcast address for the subnet.
172.16.25.12 is in the 172.16.25.8 subnet and uses broadcast address 172.16.25.15.
You're finished: