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TOPIC: Finding the matching route on a router - please help

Finding the matching route on a router - please help 7 years 6 months ago #29735

  • Damien610
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Predict which route the router would match for packets to the following addresses
172.16.1.1, 172.16.1.2, 172.16.2.2 and 172.16.4.3

routing table is...
R 172.16.1.1/32
R 172.16.1.0/24
R 172.16.0.0/22
R 172.16.0.0/16
R 0.0.0.0/0

The answers are
172.16.1.1 - 172.16.1.1/32
172.16.1.0 - 172.16.1.0/24
172.16.2.2 - 172.16.0.0/22
172.16.4.3 - 172.16.0.0/16

:?: Can someone please, please explain these answers in plain english before I have a nervous breakdown :?:

kind regards

piatti
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Re: Finding the matching route on a router - please help 7 years 6 months ago #29745

  • TheBishop
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Each entry in a routing table provides a routing choice that can be used when the incoming packet meets a specified condition.
In this case the condition for each routing statement is whether the destination address for the incoming packet falls within a particular IP network, and they have given you some test addresses and asked you to say which routing rule would be applied to each one.
So what you have to do is look at each routing statement in turn and use the submet mask supplied to expand it out to find the range of IP addresses it includes. Once you've done that it will be easy to see which of the routing statements will apply to each of the test addresses.
The final twist in the tail is that if there is more than one routing statement that could apply to a given address then we can assume that it is the most specific (narrowest) match that will be used. Hence 172.16.1.1 falls within multiple routing statement ranges, but the most specific one to that address is 172.16.1.1/32 so that is the one that will be preferred.
Does that make sense?
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Re: Finding the matching route on a router - please help 7 years 6 months ago #29773

  • Damien610
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Hi, thanks for you answer, it sort of makes things a bit clearer

one more question

why cant 172.16.4.3 fit in the 172.16.0.0/22 :?:

instead of 172.16.0.0/16

kind regards

Damien
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Re: Finding the matching route on a router - please help 7 years 6 months ago #29786

  • Damien610
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Predict which route the router would match for packets to the following addresses
172.16.1.1, 172.16.1.2, 172.16.2.2 and 172.16.4.3

routing table is...
R 172.16.1.1/32
R 172.16.1.0/24
R 172.16.0.0/22
R 172.16.0.0/16
R 0.0.0.0/0

The answers are
172.16.1.1 - 172.16.1.1/32
172.16.1.0 - 172.16.1.0/24
172.16.2.2 - 172.16.0.0/22
172.16.4.3 - 172.16.0.0/16

Can someone please, please explain these answers in plain english before I have a nervous breakdown

Hi Bishop, surely 172.16.4.3 can be routed via 172.16.0.0/22 as the subnet goes up by the power of 4, i.e 0, 4, 8, 12 etc...

so 172.16.4.3 would be in the 172.16.4.0 ?

sorry to be a pain, i'm very confused..... :?:
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Re: Finding the matching route on a router - please help 7 years 6 months ago #29793

  • Chojin
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Predict which route the router would match for packets to the following addresses
172.16.1.1, 172.16.1.2, 172.16.2.2 and 172.16.4.3

routing table is...
R 172.16.1.1/32
R 172.16.1.0/24
R 172.16.0.0/22
R 172.16.0.0/16
R 0.0.0.0/0

The answers are
172.16.1.1 - 172.16.1.1/32
172.16.1.0 - 172.16.1.0/24
172.16.2.2 - 172.16.0.0/22
172.16.4.3 - 172.16.0.0/16

Can someone please, please explain these answers in plain english before I have a nervous breakdown

Hi Bishop, surely 172.16.4.3 can be routed via 172.16.0.0/22 as the subnet goes up by the power of 4, i.e 0, 4, 8, 12 etc...

so 172.16.4.3 would be in the 172.16.4.0 ?

sorry to be a pain, i'm very confused..... :?:


do you understand what the /22 means?
CCNA / CCNP / CCNA - Security / CCIP / Prince2 / Checkpoint CCSA
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Re: Finding the matching route on a router - please help 7 years 6 months ago #29800

  • Damien610
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yes /22 is the same as 255.255.252.0 which means the subnet go up by the power of 4.
so the subnets will be
0
4
8
12
16
20
24
etc
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