How to configure Prefix-List on Cisco Router


(Rene Molenaar) #1

This topic is to discuss the following lesson:


(system) #2

Hello,
in the lesson is:
What about 10.60.0.0 /19? It falls within the 10.0.0.0 /8 range but it is not permitted because it has a subnet mask of /23…our ge operator says it should be /20 or larger.

But the network has mask of /19 and not /23 so it should be “it has a subnet mask of /19…”


(Rene Molenaar) #3

Hi Towdie,

You are totally right, just fixed this typo. Thanks for pointing it out!

Rene


(system) #4

Rene i really like the way you explain.Thank you so much…


(Rene Molenaar) #5

Yes if you use a prefix-list without GE or LE then it’s just the prefix that you matched. For example, 172.16.1.0/24 is the same as 172.16.1.0 0.0.0.255 in an access-list.


(system) #6

Rene- Its crisp & Clear explanation…however can you help me to understand what prefixes are covered for 128.0.0.0/3 & 128.0.0.0/5…?


(Rene Molenaar) #7

Hi Raghav,

First you need to figure out what /3 and /5 is, write down the subnet mask:

/3 = 224.0.0.0
/5 = 248.0.0.0

A quick way to find out how many subnets are covered is by taking the number 256 and subtracting the subnet mask:

256 - 224 = 32 subnets
256 - 248 = 8 subnets

So for /3 it looks like this:

Subnet #1: 0.0.0.0/3
Subnet #2: 32.0.0.0/3
Subnet #3: 64.0.0.0/3
Subnet #4: 96.0.0.0/3
Subnet #5: 128.0.0.0/3
Subnet #6: 160.0.0.0/3

And for /5:

Subnet #1: 0.0.0.0/3
Subnet #2: 8.0.0.0/3
Subnet #3: 16.0.0.0/3
Subnet #4: 24.0.0.0/3
Subnet #5: 32.0.0.0/3
Subnet #6: 40.0.0.0/3
Subnet #7: 48.0.0.0/3
Subnet #8: 56.0.0.0/3
Subnet #9: 64.0.0.0/3
Subnet #10: 72.0.0.0/3
Subnet #11: 80.0.0.0/3
Subnet #12: 88.0.0.0/3
Subnet #13: 96.0.0.0/3
Subnet #14: 104.0.0.0/3
Subnet #15: 112.0.0.0/3
Subnet #16: 120.0.0.0/3
Subnet #17: 128.0.0.0/3
Subnet #18: 136.0.0.0/3
Subnet #19: 144.0.0.0/3

Now you can see what prefixes are covered :slight_smile:

Rene


(system) #8

Thanks for this explanation. It is really clear like the rest of all the subjects you covered.


(system) #9

Just a quick typo, nothing serious…

10.1.0.0/16 is permitted because it’s in the 10.0.0.0/8 range and has a /16 subnet mask.
10.4.0.0/16 is permitted because it’s in the 10.0.0.0/8 range and has a /19 subnet mask.

Should be a /19 instead of /16


(ALFREDO V) #10

Hi Rene,

What about route-maps lesson?. It would have been nice to know how to apply it on different cases. Just a thought.

Love your lessons. I am stick to it


(Rene Molenaar) #11

Hi Alfredo,

Hmm i’ll think about it. Route-maps can be used for many different things so not sure if 1 lesson with all these mixed topics would work. Here are some examples where we use route-maps:








Rene


(Jude O) #12

Rene,

Do you have any video on subnetting ?


(Rene Molenaar) #13

Hi Jude,

All subnetting material can be found here:

Rene


(Frades) #14

Hi Rene, you explained it clearly, just one question. you said that
“ip prefix-list NAME permit 0.0.0.0/0 le 32” is like permitting all right?
is this the same with
“ip prefix-list NAME permit 0.0.0.0/0 ge 1” ?


(Rene Molenaar) #15

Hi John,

Good question, it’s almost the same…the only difference is that 0.0.0.0/0 ge 1 won’t allow a default route.

The ge 1 means that the prefix should have a /1 or larger and a default route has a /0.

Rene


(Frades) #16

rene, if i have this prefix list “0.0.0.0/3 ge 24”, so anything on the 1st octet that is 0-31 is a valid range?


(Rene Molenaar) #17

Hi John,

It means the first 3 bits of the 1st octet have to be a 0 and the subnet mask should be /24 or larger.

00000000 = 0
00011111 = 31

So yes, you are right :slight_smile:

Rene


(Hussein Samir) #18

Hi Rene,

What is the benefit of using seq number in Prefix-List ?

Thanks.


(Mario R) #19

I know this should be a clear answer, but for some reason I’m not understanding how 128.0.0.0/2 covers the whole Class B network range. If I write out 128 into decimal, it will be: I0000000. Now the mask says to match on the first two bits, so the only match would be the I0 ( I is matched and 0 is not matched?) which again equals to only the 128 network range. I would like a clear explanation on this if you may. Thanks


(Rene Molenaar) #20

Hi Mario,

The first two bits have to be 10 so that means the following range is class B:

10000000 - 10111111

Or in decimal:

128 - 191

Rene