I'll try to explain you:
VLSM Overlapping considers the situation in which some ip addresses that are
in a subnet, are also in another subnet, so the subnets overlaps and it should
For example consider the following two subnets: 10.8.100.96.0/20 and 10.8.104.0/24
The first subnet has this usable ip addresses range: 10.8.96.1 to 10.8.111.254.
The second subnet has this usable ip addresses range:10.8.104.1 to 10.8.104.254
The two subnet overlaps because, for example, in the first subnet can be a host
with the ip address 10.8.104.10, but also in the second subnet can be a
host with the same address.
When you have a problem that give you some subnets (or ip addresses from which calculate subnets) and ask you to find
which ones overlaps, you have to:
order the subnets from the lowest to the highest;
find the respective subnets broadcast address;
compare if the usable ip addresses of a subnet are also in another subnet (overlaps).
An example (taken from the Official Cert Guide):
These are three ip addresses.
From the lowest to the highest:
172.20.1.222/27 subnet address: 172.20.1.192 subnet broadcast address:172.20.1.223
172.20.1.225/28 subnet address: 172.20.1.224 subnet broadcast address:172.20.1.239
172.20.1.237/30 subnet address: 172.20.1.236 subnet broadcast address:172.20.1.239
The first subnet 172.20.1.192 doesn't overlaps the second;because they are in order
it won't overlaps the third.
The second subnet 172.20.1.224 overlaps with the third 172.20.1.236, because a part of the range
of addresses in the second subnet (172.20.1.236 to 172.20.10.238) are also in the third
So, the second and the third subnet overlap.
I hope I've given you a clear answer.