I am currently working on an assignment that asks me to either aggregate four given IP addresses or state why they cannot be aggregated. I did calculate an IP address from which I believe that it summarizes the given four addresses, however, I am sure there's a catch. They wouldn't ask so shady if there wasn't. How can I find out whether I have a set of addresses that cannot be summarized? Could you provide a concrete example?
If the aggregate network is not contiguously used then aggregating the subnets would hijack and blackhole some traffic. For example:
10.1.0.0/24 - Site A
10.1.1.0/24 - Site A
10.1.2.0/24 - Site B
10.1.3.0/24 - Site A
Aggregating 10.1.0.0/22 to Site A would steal the 10.1.2.0/24 traffic from Site B and route it to Site A. That's not to say you can't aggregate the traffic, but doing so would be unwise.
Also a large disparity in the subnets would make them unfeasible to aggregate, 172.16.1.0/24 and 192.168.1.0/24 for instance.