Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

Help with Prefix Lists

Hello All,

I have been trying to understand Prefix lists. I understand why they are used, but having difficulty in understanding what networks a certain prefix list actually covers. I have pasted a couple of examples that I have come across below, and I am hoping somebody can explain what networks are covered by them;-

ip prefix-list test 1 seq 10 permit 10.0.0.0/11 ge 24

ip prefix-list test 2 seq 10 permit 10.150.0.0/16 ge 24

ip prefix-list test 3 seq 10 permit 10.128.0.0/24 ge 25

Also, if anybody knows of a good tuition page, please could you let me know

Many Thanks

Russ

Everyone's tags (2)
1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Help with Prefix Lists

Hi Russ,

ip prefix-list test 1 seq 10 permit 10.0.0.0/11 ge 24

This entry permits all subnetworks of 10.0.0.0/11 (i.e. 10.0.0.0/255.224.0.0, or any subnet in the range 10.0.0.0 - 10.31.255.255) whose netmask is at least /24.

For example, 10.10.0.0/24 would pass, 10.8.0.0/25 would pass, but 10.6.0.0/23 would not pass because the netmask is not at least /24, although this network is a subnet of 10.0.0.0/11. Also, 10.40.0.0/24 would not pass because this is no longer a subnet of 10.0.0.0/11.

ip prefix-list test 2 seq 10 permit 10.150.0.0/16 ge 24

This entry permits all subnetworks of 10.150.0.0/16 whose netmask is at least /24. The examples would be analogous as before.

ip prefix-list test 3 seq 10 permit 10.128.0.0/24 ge 25

This entry permits all subnetworks of 10.128.0.0/24 whose netmask is at least /25.

Also, if anybody knows of a good tuition page, please could you let me know

I do not know if it is good but have a look at this thread:

https://supportforums.cisco.com/message/3568328#3568328

Best regards,

Peter

2 REPLIES
Cisco Employee

Help with Prefix Lists

Hi Russ,

ip prefix-list test 1 seq 10 permit 10.0.0.0/11 ge 24

This entry permits all subnetworks of 10.0.0.0/11 (i.e. 10.0.0.0/255.224.0.0, or any subnet in the range 10.0.0.0 - 10.31.255.255) whose netmask is at least /24.

For example, 10.10.0.0/24 would pass, 10.8.0.0/25 would pass, but 10.6.0.0/23 would not pass because the netmask is not at least /24, although this network is a subnet of 10.0.0.0/11. Also, 10.40.0.0/24 would not pass because this is no longer a subnet of 10.0.0.0/11.

ip prefix-list test 2 seq 10 permit 10.150.0.0/16 ge 24

This entry permits all subnetworks of 10.150.0.0/16 whose netmask is at least /24. The examples would be analogous as before.

ip prefix-list test 3 seq 10 permit 10.128.0.0/24 ge 25

This entry permits all subnetworks of 10.128.0.0/24 whose netmask is at least /25.

Also, if anybody knows of a good tuition page, please could you let me know

I do not know if it is good but have a look at this thread:

https://supportforums.cisco.com/message/3568328#3568328

Best regards,

Peter

New Member

Help with Prefix Lists

Thanks Peter!

108
Views
0
Helpful
2
Replies