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Bernardo Oliveira's questions

Martin Hope
Bernardo Oliveira
Asked: 2011-06-06 10:22:39 +0800 CST
  • 4

I'm studying for a test and found myself in a "dead end" with a question about netmasks.

The question is:

Based on the next figure, once both sub-nets are configured, 
it's observed that hosts in a sub-net can't access hosts in the other sub-net. 
The IP's for each station and for the router are:

Station 01: 172.16.78.192
Station 02: 172.16.74.3
Station 03: 172.16.94.3
Station 04: 172.16.82.254

Router's port 01: 172.16.65.1
Router's port 02: 172.16.88.1

Figure:
[Station 01]-------------------------------[Station 02]
                            | 
                            | port 01
                         [Router]   
                            | port 02
                            |
[Station 03]--------------------------------[Station 04]  

This problem is caused because of the netmask that was chosen. 
Which of the following netmasks will allow all hosts to communicate
in a correct form?

a) 255.255.240.0    ---- **This is marked as the correct answer in the book**
b) 255.255.248.0
c) 255.255.252.0
d) 255.255.224.0    ---- **This is what I think is the correct answer**
e) 255.255.232.0

Why I think 255.255.224.0 is the correct one?

Router's port 01: 172.16.65.1
Router's port 02: 172.16.88.1

I know the netmask will end as 255.255.X.0, 
so transforming the ports IP's to binary:

Port 01: 0 1 0 0 0 0 0 1 . 0 0 0 0 0 0 0 0 = 65.0
Port 02: 0 1 0 1 1 0 0 0 . 0 0 0 0 0 0 0 0 = 88.0

For all the hosts to be able to communicate with each other, they must be in the same network, right?

So, if the mask is 255.255.224.0, we have the last 2 bytes as:

1 1 1 0 0 0 0 0 . 0 0 0 0 0 0 0 0 = 224. 0 

And doing an AND with the Router's port:

          NET  |             HOSTS
Port 01: 0 1 0 | 0 0 0 0 1 . 0 0 0 0 0 0 0 0 = 65 . 0
Port 02: 0 1 0 | 1 1 0 0 0 . 0 0 0 0 0 0 0 0 = 88 . 0
Netmask: 1 1 1 | 0 0 0 0 0 . 0 0 0 0 0 0 0 0 = 224 . 0

Resulting in: 0 1 0 0 0 0 0 0 . 0 0 0 0 0 0 0 0 = 64. 0

All hots will be in the network: 172.16.64.0, being able to communicate with each other.

Is that correct? Or the correct netmask would really need to be 255.255.240.0 and if so, why is that? I can't seem to understand that.

Thanks everyone!

ip subnet netmask