The block size method for IPv4 subnetting: interesting octet, block size, network, broadcast and host range in under ten seconds, with worked examples.
Exam subnetting is not binary arithmetic; it is one subtraction and one rounding. Here is the whole method.
Write the mask for the prefix. /8, /16 and /24 are full octets of 255. Anything in between has one octet that is neither 255 nor 0: that is the interesting octet. For /27 the mask is 255.255.255.224, so the fourth octet is interesting.
256 - 224 = 32. Subnets step by 32 in that octet: 0, 32, 64, 96, 128, 160, 192, 224. Memorise the eight mask values and their block sizes and you never do this again:
| Mask octet | 128 | 192 | 224 | 240 | 248 | 252 | 254 |
|---|---|---|---|---|---|---|---|
| Block size | 128 | 64 | 32 | 16 | 8 | 4 | 2 |
Take the address octet and round it down to the nearest multiple of the block size. For 192.168.10.77/27: 77 ÷ 32 = 2 remainder 13, so the block starts at 2 × 32 = 64. The network is 192.168.10.64.
The next block starts at 96, so the broadcast is 192.168.10.95. Hosts run from .65 to .94, which is 30 addresses: 25 - 2.
They become 0 in the network address and 255 in the broadcast. For 10.20.130.5/20 the interesting octet is the third (mask 255.255.240.0, block 16): 130 rounds down to 128, so the network is 10.20.128.0 and the broadcast is 10.20.143.255.
Each miss in Subbing shows exactly these steps for the question you got wrong. Ten minutes a day for a week is usually enough.
More on this topic: Subnetting practice, with sample questions and the full list of question types.