Category:Cisco Systems -> Routing and Switching

Say we need to filter some subnets from being distributed in or out within a dynamic routing protocol. Let’s say that we need to filter subnet **192.168.x.0/24** from being advertised, announce it in prefix-list and apply it to the distribute-list.

Before creating a prefix-list we need to know that the following rules are applied to *ge* and *le* statements which are optionally used in prefix-list syntax:

*ge*must be greater than length value. The length value is the value at the end of the prefix i.e. 192.168.0.0/16*le*must be greater or equal than the ge value. Therefore if we have a value of 19 for ge, then the value for le must be 19 or greater (up to 32)

Using the following statement as an example:

```
192.168.0.0/16 ge 19 le 24
```

The first part states that the first 16 bits of the prefix must match, that is *192.168*. The last 16 bits can be anything.

The *ge 19* means that the subnet mask for the prefix must be at least 19 bits or 255.255.224.0

The *le 24* means that the subnet mask for the prefix must be at most 24 bits or 255.255.255.0

Therefore the above statement allows prefixes that begin with 192.168 and that have a subnet mask of between 19 and 24 bits.

If we want a prefix that starts with **192.168.x.0** that has only a 24 bit mask. This would be achieved with the following statement:

```
ip prefix-list LIST deny 192.168.0.0/16 ge 24 le 24
```

ip prefix-list LIST permit 0.0.0.0/0 le 32