Hello Jenny
Hmm, I don’t believe that the area range command creates a summary route to null0. It creates a summary route for inter-area routes (LSA type 3 routes), but not to Null0. Take a look at this Cisco command reference for more info:
Now, what this command does introduce, as configured in your topology, is summarization of the 10.0.0.0/8 network. This summarization occurs as far as the advertisements from Router B into Area 1 go. So you won’t see the summarization in the routing table of Router B, but in the routing table of Router A as an IA route. The same occurs in the lesson as well.
Does that make sense?
I hope this has been helpful!
Laz