Prefix Lists

Revised July 3, 2002

Questions? Comments? E-mail: AL "at" CatSpace "dot" com


Exercises:

1. Construct a prefix list that permits only the 192.168.1.0/24 network.

2. Construct a prefix list that denies network 119.0.0.0, and permits all other prefixes (including all subnets of 119.0.0.0).

3. Construct a prefix list that permits only the default route.

4. Construct a prefix list the permits everything except the default route.

5. Construct a prefix list that permits network 172.16.0.0 and any of its subnets, and denies all other prefixes.

6. Construct a prefix list that permits only the following prefixes:

7. Construct a prefix list that:

8. Construct a prefix list that permits any prefix matching the first 20 bits of 175.29.64.0 which has a mask of at least /26 but not exceeding /29, and denies all other prefixes.

9. Construct a prefix list that denies any prefix matching the first 19 bits of 15.26.96.0 with any mask up to and including /32, and permits any other prefix.

10. Construct a prefix list that denies the RFC 1918 private networks and any of their subnets, and permits everything else.

11. Construct a prefix list that permits any subnet of network 15.0.0.0 (but not the network), and denies everything else. Your router lies within AS 65011. Place the prefix list in service in the inbound direction with BGP neighbor 1.2.3.4.

12. Construct a prefix list that denies 162.56.0.0/16 and all of its subnets (with the exception of 162.56.209.208/29, which is permitted), and permits all other prefixes. Your router lies within AS 65012. Place the prefix list in service in the outbound direction with its BGP neighbor having address 5.6.7.8.

13. Construct a prefix list that permits the CIDR block containing the thirty-two class C networks beginning with 200.202.160.0/24, and denies everything else. Your router is within AS 65013. Place the prefix list in service in the inbound direction with BGP peer-group "Lucky_13".

14. Construct a prefix list that denies any prefix for which the most-significant four bits are "0110", and permits everything else.

15. Construct a prefix list that permits the host address of "CatSpace", and denies everything else.


EXTRA CREDIT:

16. Construct a prefix list that permits only classful networks, and denies everything else.

17. Construct a prefix list that denies only supernets, and permits everything else.

18. Construct a prefix list that permits only subnets, and denies everything else.

19. Construct a prefix list that permits only CIDR blocks encompassing at least 32 class-C equivalents.

20. Construct a prefix list that permits only the RFC 1918 private networks and their subnets, and configure RIP to use this prefix list for outbound routing advertisements.


ANSWERS:

1. The prefix list is:

 ip prefix-list test1 seq 5 permit 192.168.1.0/24

2. The prefix list is:

 ip prefix-list test2 seq 5 deny 119.0.0.0/8
 ip prefix-list test2 seq 10 permit 0.0.0.0/0 le 32

3. The prefix list is:

 ip prefix-list test3 seq 5 permit 0.0.0.0/0

4. The prefix list is:

 ip prefix-list test4 seq 5 deny 0.0.0.0/0
 ip prefix-list test4 seq 10 permit 0.0.0.0/0 le 32

5. The prefix list is:

 ip prefix-list test5 seq 5 permit 172.16.0.0/16 le 32

6. The prefix list is:

 ip prefix-list test6 seq 5 permit 10.2.8.32/27 le 30

7. The prefix list is:

 ip prefix-list test7 seq 5 deny 197.25.94.192/26
 ip prefix-list test7 seq 10 deny 197.25.94.240/28
 ip prefix-list test7 seq 15 deny 197.25.94.252/30
 ip prefix-list test7 seq 20 deny 198.82.0.0/16
 ip prefix-list test7 seq 25 permit 0.0.0.0/0 le 32

8. The prefix list is:

 ip prefix-list test8 seq 5 permit 175.29.64.0/20 ge 26 le 29

9. The prefix list is:

 ip prefix-list test9 seq 5 deny 15.26.96.0/19 le 32
 ip prefix-list test9 seq 10 permit 0.0.0.0/0 le 32

10. The prefix list is:

 ip prefix-list test10 seq 5 deny 10.0.0.0/8 le 32
 ip prefix-list test10 seq 10 deny 172.16.0.0/12 le 32
 ip prefix-list test10 seq 15 deny 192.168.0.0/16 le 32
 ip prefix-list test10 seq 20 permit 0.0.0.0/0 le 32

11. The prefix list is:

 ip prefix-list test11 seq 5 permit 15.0.0.0/8 ge 9
To place it in service:
 router bgp 65011
  neighbor 1.2.3.4 prefix-list test11 in

12. The prefix list is:

 ip prefix-list test12 seq 5 permit 162.56.209.208/29
 ip prefix-list test12 seq 10 deny 162.56.0.0/16 le 32
 ip prefix-list test12 seq 15 permit 0.0.0.0/0 le 32
To place it in service:
 router bgp 65012
  neighbor 5.6.7.8 prefix-list test12 out

13. The prefix list is:

 ip prefix-list test13 seq 5 permit 200.202.160.0/19
To place it in service:
 router bgp 65013
  neighbor Lucky_13 prefix-list test13 in

14. The prefix list is:

 ip prefix-list test14 seq 5 deny 96.0.0.0/4 le 32
 ip prefix-list test14 seq 10 permit 0.0.0.0/0 le 32

15. The "hardest" part of this problem (and it isn't very hard!) is determining the IP address of "CatSpace". The easiest way to do that is to either "ping" or "trace" to "www.catspace.com" from any Internet-connected host, and let DNS resolve the address (which turns out to be 64.82.100.67). The prefix list is therefore:

 ip prefix-list test15 seq 5 permit 64.82.100.67/32

16. The prefix list is:

 ip prefix-list test16 seq 5 permit 0.0.0.0/1 ge 8 le 8
 ip prefix-list test16 seq 10 permit 128.0.0.0/2 ge 16 le 16
 ip prefix-list test16 seq 15 permit 192.0.0.0/3 ge 24 le 24

17. A "supernet" is any block that contains more than one classful network. The prefix list is:

 ip prefix-list test17 seq 5 deny 0.0.0.0/1 le 7
 ip prefix-list test17 seq 10 deny 128.0.0.0/2 le 15
 ip prefix-list test17 seq 15 deny 192.0.0.0/3 le 23
 ip prefix-list test17 seq 20 permit 0.0.0.0/0 le 32

18. The prefix list is:

 ip prefix-list test18 seq 5 permit 0.0.0.0/1 ge 9
 ip prefix-list test18 seq 10 permit 128.0.0.0/2 ge 17
 ip prefix-list test18 seq 15 permit 192.0.0.0/3 ge 25

19. Since a "class-C equivalent" prefix has a "/24" mask, a block of thirty-two of them would have a "/19" mask (moved five bits to the left). The default route is not considered a "CIDR block". The prefix list is:

 ip prefix-list test19 seq 5 deny 0.0.0.0/0
 ip prefix-list test19 seq 10 permit 0.0.0.0/0 le 19

20. The prefix list is:

 ip prefix-list test20 seq 5 permit 10.0.0.0/8 le 32
 ip prefix-list test20 seq 10 permit 172.16.0.0/12 le 32
 ip prefix-list test20 seq 15 permit 192.168.0.0/16 le 32
To place it in effect for outbound RIP updates:
 router rip
  distribute-list prefix test20 out
Note: My testing showed that prefix lists worked as expected with BGP, EIGRP, IGRP and RIP. The results with IS-IS and OSPF varied by IOS version.


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Copyright © 2002 Alchemy, Inc. All rights reserved.