# Configuring a prefix list

Use this procedure to configure a prefix list to allow or deny specific route updates.

## About this task

A prefix list policy specifies route prefixes to match. After a match occurs, the system uses the route.

The prefix list contains a set of contiguous or noncontiguous routes. Reference prefix lists by name from within a routing policy.

## Procedure

1. In the navigation tree, open the following folders: Configuration > IP.
2. Click Policy.
3. Click the Prefix List tab.
4. Click Insert.
5. In the ID box, type an ID for the prefix list.
6. In the Prefix box, type an IP address for the route.
7. In the PrefixMaskLength box, type the length of the prefix mask.
8. Configure the remaining parameters as required.
9. Click Insert.

## Prefix List field descriptions

Use the data in the following table to use the Prefix List tab.

Name | Description
--- | ---
ID | Configures the list identifier.
Prefix | Configures the IP address of the route.
PrefixMaskLen | Configures the specified length of the prefix mask. You must enter the full 32-bit mask to exact a full match of a specific IP address, for example, if you create a policy to match on the next hop.
Name | Names a specified prefix list during the creation process or renames the specified prefix list. The name length can use from 1–64 characters.
MaskLenFrom | Configures the lower bound of the mask length. The default is the mask length. Lower bound and higher bound mask lengths together can define a range of networks.
MaskLenUpto | Configures the higher bound mask length. The default is the mask length. Lower bound and higher bound mask lengths together can define a range of networks.