Hello Professor Guo,

I think there is a wrong answer given in Homework 2 solution question 3. I found the same question in our power point slides 

I found below right answer.

Here is an example of what your answer should look like, beginning with the network on E0 and working through to E3:

Router(config)#access-list 1 deny 172.16.128.0 0.0.31.255

Router(config)#access-list 1 deny 172.16.48.0 0.0.15.255

Router(config)#access-list 1 deny 172.16.192.0 0.0.63.255

Router(config)#access-list 1 deny 172.16.88.0 0.0.7.255

Router(config)#access-list 1 permit any

Router(config)#interface serial 0

Router(config-if)#ip access-group 1 out

So, Can you please recheck the solution?

Yeah, i also have same problem

Yes I also have same problem