Actually, it doesn't appear that you are having problems with the prefix-lists but more how subnetting and supernetting work. For a prefix list of 30.0.0.0/7 which is the same as 30.0.0.0 254.0.0.0 or reverse mask of 1.255.255.255, it matches up as the following: 30 = 0001111031 = 00011111mask11111110and= 00011110 for 30 and 31 So 30 with a mask of 7 bits has the same "and" as 31 which is 00011110 since we aren't checking the eighth bit. For the /14 example you have, 30.0.0.0/14 will match 30.0.0.0, 30.1.0.0, 30.2.0.0 and 30.3.0.0 since we are checking the entire first octet and 6 bits of the second octet while we don't care about the last two bits of the second octet. When you add a less than or equal to the prefix, this doesn't change what the actual subnets are matched, this states what prefix lengths will be accepted. So, the le 16 on your example says that it will match any prefix of the above four provided that the subnet mask of the prefix is 14, 15 or 16 bits long. Because of this, 30.0.0.0/16 will be matched, but 30.0.0.0/8 will not be matched.
--- On Thu, 2/2/12, unsobill <[email protected]> wrote: From: unsobill <[email protected]> Subject: [OSL | CCIE_RS] Prefix-list logic To: [email protected] Date: Thursday, 2 February, 2012, 1:20 AM Greets, How can i ease my pain in understanding logic behind prefix lists. It feels sometimes i reread manual on prefix-list see some easy examples like ip prefix-list deny 0.0.0.0/8 le 32 -blocking ALL class A addresses with subnets 255.255.255.255 through 255.0.0.0.0 correct ? ...and I move on... only to find weird cases that i cant explain like this one : ip prefix-list deny 30.0.0.0/7 -apparently blocking 30.0.0.0 and 31.0.0.0.0 ip prefix-list deny 30.0.0.0/14 le 16 -same results ? ? ? Should i use some formula when faced with prefix-lists ? How do you personally approach and structure them ? Thanks _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com Are you a CCNP or CCIE and looking for a job? Check out www.PlatinumPlacement.com http://onlinestudylist.com/mailman/listinfo/ccie_rs _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com Are you a CCNP or CCIE and looking for a job? Check out www.PlatinumPlacement.com http://onlinestudylist.com/mailman/listinfo/ccie_rs
