Wild Card Mask ?????

Geeljire

first off hi ppl, I'm new to the forum just registered today !

currently doing CCNA, whil studying Access Control List i got stumped ....


I had a hard enough time, understanding the nitty gritty of sub netting & VLSM, but wild card Mask , i'm confused

anybody can help out with this, or point me to some good material online, would appreciate it ....

lordy

Hi and welcome to the forum !

The wild card masks can be confusing first but in fact they are pretty easy to handle.

Let's assume you have an ACL that should match on the network 1.2.3.0/24.

The subnet mask would then of course be 255.255.255.0.

Wild card masks are simply inverted subnet masks. The wild card mask for this is 0.0.0.255.

To make the inveriton a little clearer here is another example.

If the subnet mask would be 255.255.192.0 the wildcard mask would be 0.0.63.255.

Hope this helps.

Regards,
Lordy

dmaftei

A wildcard mask is not an inverted subnet mask. A wildcard mask is a bit pattern that indicates what bits should be compared when you try to "match" two IP addresses. With wildcard mask 0.0.0.255 you would compare the first 24 bits and ignore the last 8 bits; with wildcard mask 255.0.0.0 you would ignore the first 8 bits and compare the last 24 bits.

Now consider wildcard mask 255.0.0.0. The inverse of that (presumably a subnet mask) is 0.0.0.255; but 0.0.0.255 is not legal as a subnet mask (you already know that).

pipearce

Widcard Subnet Masks?! This is the first time I've ever heard of this. Here is a good article explaining what this is all about.

http://www.rhyshaden.com/ipadd.htm

It appears Lordy is correct with his example and the article describes it as "don't care bits" being represented by 1's, which is kind of the opposite of how things are normally done. This is used in ACLs on Cisco Routers mainly.

There's my new thing learned today!

Cheers, Paul

dmaftei

pipearce wrote:

There's my new thing learned today!

You may be better off going directly to the source... Search for "contiguous" on the following page: http://www.cisco.com/en/US/products/ps6350/products_command_reference_chapter09186a0080446247.html