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ACL Wildcard Masking...

Hello everyone, I am reading the Cisco 5.2.3 ACL WIldcard Masking chapter and I have come across a stump....something that I cannot get myself to understand.

With the image that I've attached, you will see that cisco is using the 192.168.10.0 IP and a wildcard mask of 0.0.255.255.  I am guessing it's safe to say that with the inverse mask being 0.0.255.255, that the subnet would be 255.255.0.0 thus being a /16 subnet.

My problem is understanding why Cisco choose to use 0.0.255.255 for 192.168.10.0.  Why didn't they use 0.0.0.255?

[IMG]http://img.photobucket.com/albums/v154/mugen85r/1681ef06.png[/IMG]

I have asked a few others but it just gets me more and more confused.  

Can somebody explain to me why they used 0.0.255.255 for the wildcard mask? why not use 0.0.0.255?

thank you!

3 REPLIES
Cisco Employee

ACL Wildcard Masking...

mrrhtuner,

From what I can see from the image it shows an IP address of 192.168.10.0 which is a valid IP address with the mask of a /16(255.255.0.0). It would not be a valid IP address if it was a /24(255.255.255.0) since that would be a host address which is the reason for the 0.0.255.255 wild card mask they choose to show. Hope this helps.

Ryan

New Member

ACL Wildcard Masking...

Thank you Ryan, so maybe I am getting confused with this...

When you state 192.168.10.0 is a valid IP address with a mask of /16...what makes it that?

when looking at 192.168.10.0, how can I see that it is a valid address with a /16?  I belive that is why I am getting confused.

Cisco Employee

ACL Wildcard Masking...

mrrhtuner,

I'm not sure how familiar you are with subnetting but I'll try to make this hopefully easy to follow...

So for 192.168.10.0 with a /16(255.255.0.0) mask you would have a valid IP range of..

192.168.0.0-host(unusable)

192.168.0.1-usable

through

192.168.255.254-usable

192.168.255.255-broadcast(unusable)

so 192.168.10.0 would fall within that range of usuable.

For 192.168.10.0 with a /24(255.255.255.0) mask you would have a valid range of..

192.168.10.0-host(unusable)

192.168.10.1-usuable

through

192.168.10.254-usuable

192.168.10.255-broadcast(unusable)

So from the above example you can see that 192.168.10.0 would not be a valid ip address with a /24. If it was 192.168.10.1 then it would be and then the picture you posted before might show the wildcard mask as 0.0.0.255.

I can see how the picture can be confusing.

Hope this clears things up.

Ryan

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