Longest prefix match routing - How to find right answer

[LetMePass]

Hi guys,

I'm currently reviewing the longest prefix match routing lesson, but I'm hitting a snag. Let me explain.

I know the following thing: The entry that has the longest number of network bits that match the ip destination address is always the best match.

1/ First exercise

Easy.

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2/ Second exercise

Even easier, ExSim-Max kind of help you here with additional information in the multiple answers.

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3/ Third exercise

Wait.. what? It's /24 and not 28/? I don't really get it.. Is there any formula to find out the right answer? Am I supposed to perform subnetting?

I welcome any comment, please.

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EDIT.

Here what I might have missed:

1/ First exercise

Destination -> 192.168.1.1

.1=.00000001
/24=.00000000 X
/30=.00000000 V

2/ Second exercise

Destination -> 10.20.0.14

.14=.00001110
/26=.00000000 X
/28=.00000000 V

3/ Third exercise

Destination -> 10.20.0.17

.17=.00010001 
/24=.00000000 V
/28=.00000000 X

Would you agree on my demonstration above?

[M02@rt37 · Accepted solution]

Hello @LetMePass,

ip host = 10.20.0.17.

If /28 as mask, the subnet of that host should be 10.20.0.16. The ip host 10.20.0.17 is not part of the subnet 10.20.0.0/28 ; with this subnet you have host from .1 to .14 (.15 as broadcast).

Since on the RIB you have route towards 10.20.0.0, answer C is the good answer.

[LetMePass]

Hello,

Yes, you are correct, but I remains unsure about the right formula to use. Boson gives a similar explanation with real way to find out right answer. What would be the fastest way of calculating that?

I would say:

ip host = 10.20.0.17.

/24=255.255.255.0 | 255-0=255 

/28=255.255.255.240 | 255-240=15

/24 last broadcast=10.20.0.255

/28 last broadcast=10.20.0.15

[M02@rt37]

Hello @LetMePass,

Yes!

That's also explain the third exercice...host 10.20.0.17 is not part of 10.20.0.0/28 ! It is part of 10.20.0.16/28.

[MHM Cisco World]

Destination -> 192.168.1.1

the bits formal is 
Destination IP bit >= the route mask
 

Thanks A Lot
MHM