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Understanding wildcard

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Many people have this question. Since there is already a subnet mask, why should wildcard masks be used and what is the difference between them?

The short answer is the following:


There is a basic rule for the wild card mask: '0' in the wildcard mask indicates MUST MATCH while '1' means IGNORE, '0' and '1' can be discontinuous. 

So, the wildcard mask can match the addresses that are not on a subnet boundary, being more flexible than the subnet mask.

Let's take some examples.

Generally, the wildcard/subnet mask is used in the following scenarios:

1. A single IP address:

  Subnet mask:

  Wildcard mask:

2. A subnet

  Subnet mask:

  Wildcard mask:

In both cases, they can implement the same functions.

3. A range of IP addresses

In this case we can see there difference.

Assume that the device needs to allow the IP address、… to pass through.

Subnet mask: can't represent

Wildcard mask:

The wildcard mask also has insufficient. It cannot accurately match the mask information.

For example, we want allow to pass through and try to use ACL:

ACL 2000

   Rule permit source

When applying this ACL, not only, but also,… can match the ACL.

That's why in practice, we usually use ip-prefix instead of ACL to match the routing information.

  • x
  • convention:


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