networking - Explanation needed for a subnet question from a quiz(see image)

Question

I am learning about subnets and doing some quizzes. I don't understand one of the questions and it doesn't explain it either. Question screenshot with answer

Can someone explain why the answer is 6 and not 1? Because you have 254 usable IP addresses and they can all fit into 1 subnet, right?

question from:https://stackoverflow.com/questions/65872584/explanation-needed-for-a-subnet-question-from-a-quizsee-image

Answer 1

The reason why the answer is 6 is written in the output provided. In order to route between different ip networks you need to cross "transit" links, which in basic IP networks must be also addressed. Your response was correct (4) for the amount of subnets needed for hosts hosted behind those routers, but then you have two "uplinks" between those 3 routers that you need to account subnets for.

It's a bit of a misleading question though, because those "uplinks" can also be unnumbered (not having p2p IP addresses). I would have also answered 4.