Help with Prefix Lists

[r.gasper]

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

[Peter Paluch · Accepted solution]

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

[r.gasper]

Thanks Peter!