Finding the matching route on a router - Help

Can someone help explain this to me? I am having trouble understanding this from the book.

R 172.16.0.0/22
R 172.16.0.0/16
R 0.0.0.0/0

Destination address 172.16.2.2 uses route 172.16.0.0/22

Destination address 172.16.4.3 uses route 172.16.0.0/16

Any help would be great. This is in the CCENT book pg 483 & 484.

Jeff

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Comments

mikej412

You're not the only one having problems with this

Check out this thread
http://www.techexams.net/forums/viewtopic.php?t=34261

jkane

Thanks
I tried looking for past posts on this but did not have any luck...

Thanks again
Jeff

gojericho0

Which part are you having problems understanding?

LBC90805

It will probably help you, as it did me, to convert from decimal to binary to figure this one out.

Markey

Hello JK,

I've been struggling with this also. The post within this thread explained it nicely.

172.16.0.0/22 /22 = 255.255.252.0 If you've been studying Odom's book that subnet has a magic number of 4 therefore the first subnet would be:

172.16.0.0 with a range from 172.16.0.1 thru 172.31.3.254 172.16.2.2 falls in this range

172.16.4.3 is in the next subnet 172.31.4.0 but if you use the /16 mask:

172.16.0.0/16 uses a 255.255.0.0 or a natural mask so it's host valid range is:

172.16.0.1 thru 172.16.255.254 both addresses fall into this range but because the router uses the prefix with the longer length it uses /22 for 2.2 but it can't for 4.3 because it's out of that subnet.

At least that's the way I understand it hope I'm not steering you wrong.

Markey

Every time I think I've got this figured out I find another gotcha. Here's the Boson exam question that confuses me:

IP classless is globally configured

172.16.0.0/16 is variably subnetted, 5 subnets, 4 masks
R 172.16.1.1/32 [120/1] via 172.16.25.2, 00:00:04, Serial0/1/1
R 172.16.1.0/24 [120/2] via 172.16.25.129, 00:00:09, Serial0/1/0
R 172.16.0.0/22 [120/1] via 172.16.25.2, 00:00:04, Serial0/1/1
R 172.16.0.0/17 [120/2] via 172.16.25.2, 00:00:04, Serial0/1/0
R 0.0.0.0 [120/3] via 172.16.25.2, 00:00:04, Serial0/1/1

a. Packets sent to 172.16.130.1 use s0/1/1
b. Packets sent to 172.16.10.1 use s0/1/1
c. Packets sent to 172.16.1.11 use s0/1/0
d. Packets sent to 10.1.1.1 are discarded
e. Packets sent to 172.16.125.1 use s0/1/1
f. Packets sent to 172.16.3.1 use s0/1/1

I see it as this:

a. matches the /22, /17 and default route but /22 has the longest prefex; it's network range is 172.16.128.1-172.16.131.254

b. matches the /24, /22, /17 and default route but / 22 has the longest prefex; it's network range is 172.16.8.1-172.16.11.254

c. matches the /22, /17 and default route but / 24 has the longest prefex; it's network range is 172.16.1.1-172.16.1.254

d. matches only the default route it's packets won't be discarded

e. matches the /22, /17 and default route but / 22 has the longest prefex; it's network range is 172.16.128.1-172.16.131.254

f. matches the /22, /17 and default route but / 22 has the longest prefex; it's network range is 172.16.0.1-172.16.3.254

so the only false statement is 'd'

Boson lists the true statements as 'a,c,f'

Where am I going wrong here? This is driving me crazy!!!

suffah

I think you're looking at these wrong (or maybe I'm wrong hehe).

For example, in example b you said it's subnet range is 172.16.8.1-172.16.11.254. That's correct but the routing table doesn't have 172.16.8.0/22 listed, it is listing 172.16.0.0/22.

Markey

The way I'm lookikng at is:

172.31.0.0/22 has a subnet mask of 255.255.252.0

So the subnets would be

172.31.0.0
172.31.4.0
172.31.8.0
172.31.12.0
172.31.16.0

etc

Example B (172.31.10.1) would be in subnet 172.31.8.0 range (172.31.8.1 - 172.31.11.255)

suffah

Markey wrote:

So the subnets would be

172.31.0.0
172.31.4.0
172.31.8.0
172.31.12.0
172.31.16.0

etc

Maybe someone else can chime in but for the scenario I quoted above this should be in the routing table:

R 172.31.0.0/22 [120/1]
R 172.31.4.0/22 [120/1]
R 172.31.8.0/22 [120/1]
R 172.31.12.0/22 [120/1]
R 172.31.16.0/22 [120/1]
etc

instead of just one entry:

R 172.31.0.0/22 [120/1]

Markey

Then how do you explain the answers Boson list as correct A, C and F

suffah

I'll explain all the answers.

You are confusing *possible* subnets that fit a netmask vs the actual network listed.

Answer A:

172.16.130.1 does not fit in any of the networks in the routing table. Therefore it will use the default route.

Answer B:

172.16.10.1 network can be found in the routing table:
R 172.16.0.0/17 [120/2] via 172.16.25.2, 00:00:04, Serial0/1/0

Answer C:

172.16.1.11 network can be found in the routing table:
R 172.16.1.0/24 [120/2] via 172.16.25.129, 00:00:09, Serial0/1/0

Answer

10.1.1.1 will also use the default route.

Answer E:
172.16.125.1 network can be found in the routing table:
172.16.0.0/17 [120/2] via 172.16.25.2, 00:00:04, Serial0/1/0

Answer F:

172.16.3.1 network can also be found in the routing table:
R 172.16.0.0/22 [120/1] via 172.16.25.2, 00:00:04, Serial0/1/1
R 172.16.0.0/17 [120/2] via 172.16.25.2, 00:00:04, Serial0/1/0

It will use the /22 network.