VLSM - what do you do with the unused IPs ? are we able to group / reuse them ? - brainstorm

Question

Hi all,
At the end of all the VLSM subnetting over at the thread below.
http://computernetworkingnotes.com/subnetting-supernetting-and-vlsm/vlsm.html
The user is left with 20 ips unused; on his very last VLSM is 192.168.1.224/30 it gaves him another 8 more subnets of 2 usable hosts each.
In which he has only utilize 3 of those, leaving behind 5 subnets left (20 ips)
3 of the subnets used below
Subnet
Subnet 57
Subnet 58
Subnet 59
Network ID
224
228
232
First host
225
229
233
Last host
226
230
234
Broadcast ID
227
231
235
q1) Is there anyway whereby instead of having left with the choice to use the 5 subnets each with 2 usable hosts for future usages, is there any way i can consolidate the remaining 5 subnets into a larger subnet with more hosts ?
if i set a new subnet with 192.168.1.240/29, will there be any conflict ? as some of the other subnet  192.168.1.232/29 will be in conflict with the subnet 59 as shown above, but i will not be using them. I will only use 192.168.1.240/29.
So far from VLSM, i am seeing it as taking a subnet and break it further to smaller subnet , but can we also like group the remaining smaller subnets at the end into a bigger one ?
Regards,
Noob

Answer 1

So the starting point is 192.168.1.236.
Try applying the maths I suggested to the above eg.
255.255.255.192 = 256 - 192 = 64
so you know the last subnet = 192.168.1.192 which doesn't work.
255.255.255.224 = 256 - 192 = 32 ie. your subnets go up in 32s.
Here your subnets go up in 32s and if you counted from the start the last available subnet would be 192.168.1.224 which is no good.
255.255.255.240 = 256 - 240 = 16 so your subnets go up in 16s. Again if you counted from the start the last useable subnet is 192.168.1.240 which is okay because it starts after 192.168.1.236.
255.255.255.248 = 256 - 248 = 8 which will also work but you are looking for the largest subnet.
So you now know the largest subnet you can get from your remaining IPs is -
192.168.1.240 255.255.255.240
if you wanted to use the other IPs you would then need -
192.168.1.236 255.255.255.252 
ie. 256 - 252 = 4 and if you counted up from the start 192.168.1.236 is a valid subnet and the next one is 192.168.1.240 so it works.
If you entered 192.168.1.236 255.255.255.240 what that actually works out as is 192.168.1.224 255.255.255.240 which is not what you want because you have already used some of those IPs.
That is what I meant about bit boundaries.
You get used to the maths very quickly so I have been saying count up from the start but you can just as easily multiply the number in your head to work out available subnets and it does become second nature with practice.
Jon