Subnetting

[nabeelkhan8086]

I have sub netted network 172.16.0.0 into 4 sub networks.

172.16.0.0 - 172.16.0.63

172.16.0.64 - 172.16.0.127

172.16.0.128 - 172.16.0.191

172.16.0.192 - 172.16.0.255

So, here as seen in the diagram below, PC0 should not communicate with PC10 as they are in different subnets.

But, to my surprise, I am able to ping PC10 from PC0 and vice versa.

Could someone explain?

[Georg Pauwen]

Hello,

in your drawing, you have /18 subnet masks ? That won't work, you need /26 masks (255.255.255.192).

[Reza Sharifi]

So, here as seen in the diagram below, PC0 should not communicate with PC10 as they are in different subnets.

By default, communication between subnets is enabled. If you want to block communication between 2 different subnets, you need to deploy an access-list.

As Georg also noted, the mask on the drawing is incorrect.

HTH

[Richard Burts · Accepted solution]

The drawing is not specific about it but I am guessing that all switch ports are in the same vlan. And while the original poster suggests that there are 4 subnets, if the mask is /18 then in fact there is only a single subnet and all hosts are in the same vlan and same subnets and communication from host to host would be expected.

If the mask is changed from /18 to /26 then there would be 4 subnets. As long as all hosts are in the same vlan it is possible that they could still continue to communicate with each other. Especially if the default gateway of the host was not correctly configured host to host communication would be possible. 

If you really want to be sure that each of the 4 subnets is isolated then the switch needs to be configured with 4 vlans and hosts connected in the appropriate vlan.

[edit] We tend to assume a one to one relationship of vlan and subnet (each individual vlan has one subnet). When that assumption is not correct then unexpected behavior may result - for example 4 subnets all in the same vlan may allow host to any host communication.

[Richard Burts]

I am glad that our explanations have been helpful. Thank you for marking this question as solved. This will help other participants in the community to identify discussions which have helpful information. This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.