Prediction from routing table
xenodamus
Member Posts: 758
in CCNA & CCENT
I'm taking this from the Odem ICND1 Exam Cert Guide p. 484 is anyone is reading that as well.
The question asks you to predict the route from the following routing table. The last 3 routes are all that matter to me right now and they are:
R 172.16.0.0/22 [120/1] via 172.16.25.2, 00:00:04, S 0/1/1
R 172.16.0.0/16 [120/2] via 172.16.25.129, 00:00:09, S 0/1/0
R 0.0.0.0/0 [120/3] via 172.16.25.129, 00:00:09, S 0/1/0
The 2 IP's im concerned with are 172.16.2.2 and 172.16.4.3
The text says 172.16.2.2 will use the 1st route above because it has the longest matching prefix. Got that. But then it says the 172.16.4.3 only matches the last 2 routes and that it would choose 172.16.0.0 for the same reason.
Why wouldnt 172.16.4.3 choose the 1st route like the previous IP? Wouldnt it fall in the 172.16.4.0/22 subnet? The next command runs #show ip route 172.16.4.3 and returns 172.16.0.0 as well. What am I missing here?
The question asks you to predict the route from the following routing table. The last 3 routes are all that matter to me right now and they are:
R 172.16.0.0/22 [120/1] via 172.16.25.2, 00:00:04, S 0/1/1
R 172.16.0.0/16 [120/2] via 172.16.25.129, 00:00:09, S 0/1/0
R 0.0.0.0/0 [120/3] via 172.16.25.129, 00:00:09, S 0/1/0
The 2 IP's im concerned with are 172.16.2.2 and 172.16.4.3
The text says 172.16.2.2 will use the 1st route above because it has the longest matching prefix. Got that. But then it says the 172.16.4.3 only matches the last 2 routes and that it would choose 172.16.0.0 for the same reason.
Why wouldnt 172.16.4.3 choose the 1st route like the previous IP? Wouldnt it fall in the 172.16.4.0/22 subnet? The next command runs #show ip route 172.16.4.3 and returns 172.16.0.0 as well. What am I missing here?
CISSP | CCNA:R&S/Security | MCSA 2003 | A+ S+ | VCP6-DTM | CCA-V CCP-V
Comments
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networker050184
Mod Posts: 11,962 Mod
172.16.0.0/22 = 172.16.0.0 - 172.16.3.255
172.16.4.3 is not contained in that network and therefore will not match the route.An expert is a man who has made all the mistakes which can be made. -
xenodamus
Member Posts: 758
Aha...I was looking at the 172.16.0.0 and thinking about all the /22 networks contained in the class B as a whole. You are correct indeed.
Edit to add: You guys are great btw. I feel bad sometimes creating an entire thread for a question that I'm sure has a simple answer that I'm just not seeing. But for those of us who have no living breathing IT people around to ask, this forum is a great place. Kudos
CISSP | CCNA:R&S/Security | MCSA 2003 | A+ S+ | VCP6-DTM | CCA-V CCP-V