If you’ve been watching closely, you may have noticed the very clear pattern that develops when attempting to determine address ranges. Now that we have the long method out of the way, we can take a look at a quick shortcut. With any given subnet mask value, there is always an associated multiple at which new subnets will start. For example, if you are using the network ID 131.107.0.0, and a subnet mask of 255.255.252.0, subnetting is clearly taking place in the third octet. When a custom value of 252 is used in a mask, new address ranges always move in multiples of 4 in the third octet. For example, the first three ranges of addresses in this example would be:
131.107.4.1 – 131.107.7.254
131.107.8.1 – 131.107.11.254
131.107.12.1 – 131.107.15.254
Of course, we could calculate these the old fashioned way, breaking everything down to binary and then working from there. However, the pattern that exists can easily be put into a table to help you recall mask values and address range multiples quickly and easily, as illustrated in this subnetting shortcut table:
| Bit | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Mask | 128 | 192 | 224 | 240 | 248 | 252 | 254 | 255 |
| Ranges Multiples | 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |
So what does this table tell you? Well, imagine that you were given the network ID 10.0.0.0 with a subnet mask of 255.224.0.0. If you used the table above, you could quickly determine the multiples at which new address ranges start. Simply look for the value associated with the custom mask in the Mask row – in this case 224. Now look in the Range Multiples row directly below 224, and you’ll notice the number 32. When a custom subnet mask of 224 is used, new ranges always start at multiples of 32 in the second octet. So, in our example, the first three ranges of addresses would be:
10.32.0.1 – 10.63.255.254
10.64.0.1 – 10.95.255.254
10.96.0.1 – 10.127.255.254
While the table above works perfectly for Class A and B address ranges, it does present a small issue for Class C address subnetting. Recall that Class C addresses use only the last octet for subnetting. The table can still be used for these addresses, but with one small adjustment – you’ll need to add 1 to the beginning of every range and subtract one from the end to account for the network ID and broadcast address.
For example, imagine the address 202.202.202.0 with a subnet mask of 255.255.255.240. In this case, according to our table, the ranges should begin at multiples of 16. This is still true, and the first three subnet IDs would be:
202.202.202.16
202.202.202.32
202.202.202.48
However, calculating the ranges means remembering that simple rule I just outlined. Add one to find the beginning of the range, and subtract one from what you might normally consider the end of the range. For example, the first range would go from 202.202.202.17 up to 202.202.202.30. This is because the “normal” end of the range (202.202.202.31) is actually the broadcast address for that subnet, where the host ID is set to all binary 1s. Altogether, the first three ranges in this example would be:
202.202.202.17 – 202.202.202.30
202.202.202.33 – 202.202.202.46
202.202.202.49 – 202.202.202.62
All thing considered, the shortcut table provides a simple and effective way of recalling the address range starting points associated with a given custom subnet mask. It should be helpful as a quick reminder if jotted down before you write your exams. Just remember that when in doubt, always go back to the binary method.
Bambang F. Indarto 