ACL Wildcard Masking...

[mrrhtuner]

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!

[Ryan Gadwood]

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

[mrrhtuner]

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.

[Ryan Gadwood]

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