I would try to do it a bit more visually & more from the bottom up:
1 - Ask what is in common between the 8 networks - hopefully get 195.168.
2-then Ask where (which octet) is the change taking place? Hopefully get answered octet 3.
Then ask what are the values in octet 3? Hopefully get answered 0 thru 7.
3- NOW ask: Ok, can you count in Binary from 0 to 7 using 8 bits?
Hopefully get this response:
- 0: 00000000
- 1: 00000001
- 2: 00000010
- 3: 00000011
- 4: 00000100
- 5: 00000101
- 6: 00000110
- 7: 00000111
4-Now ask: What do you see about the bits in these binary numbers?
--HINT: How many bits are common across all 7 addresses/numbers?
Hopefully you hear the answer "5 zeros in common".
Now ask so how many of those 5 zeros are part of the network address? -Hopefully hear the answer of 5
so then how many 1's would that be if the subnet is in binary? hopefully also hear the number 5
5- Exactly - So how many ones were in the 192.168 portion already? 16 -- and how many now when you add 5 more? 21
Perfect - THAT is your subnet mask in CIDR: /21
6-Now ask, -what does that means in BINARY for the submask in octet 3? hopefully someone says "5 ones & 3 zeros".
(or if you have beeen showing the entire 11111111. 11111111.
11111000 .00000000
- so the subnet mask bits for the 3rd octet are 1111 1000.
Now ask, what is that value in Dotted decimal notation of 11111000? 128+64+32+16+8=248
So the subnet mask is 192.168.248..0
7- But what about the summary network number? Look at the 5 network bits - since we only used up to 7 networks,
all of the first 5 network bits are 0 (since even for network 7 it would be 0000 0111 - 7.
so 0+0+0+0+0 = 0 that wan't too bad.
Your final answer so your summary network is 192.168.0.0/24