IP forwarding by matching the most specific route!

LBC90805

Just a question here and some confusion on my behalf. I will quote from the ICND2 Odom book -the new book here-; just trying to figure stuff out here.

"When a particular destination IP adress matches more than one route in a router's routing table, the router uses the most specific route - in other words, the route with the longest prefix length."

OK that much I understand.

I have a question and would like to see if I can just jar the logical reasoning out of my skull.

What route would a router match for the following IP Addresses?

Addresses as follows:
172.16.2.3
172.16.4.3

What route for each above address would a router use from the following routes?
172.16.0.0/22
172.16.0.0/16
0.0.0.0/0

Any input and help would be greatly appreciated........

peanutnoggin

I believe each route would go to the 172.16.0.0/22 because it is the longest matching route in the routing table.

~Peanut Head

srg

Kinda tricky question . Since 172.16.0.0/22 ranges from 172.16.0.0 to 172.16.3.255 that will be the prefered route to get to 172.16.2.3, but 172.16.0.0/16 will be the prefered route to get to 172.16.4.3 since that's the most specific route. 0.0.0.0/0 beeing the least specific route of them all.

Hope that helps.

LBC90805

Well this is my dilemma. Don't both routes in the rouing table match each IP? The router would use 172.16.0.0/22 instead of 172.16.0.0/16 in both cases. First 172.16.0.0/16 is not using any subnetting what so ever, 172.16.0.0/22 is! Second I don't know why 172.16.0.0/16 would be used over 172.16.0.0/22, in every instance 172.16.0.0/22 remains the most specific route. `Am I missing something here?

Thanks for everyone's input thus far

gojericho0

172.16.4.3 does not fall into 172.16.0.0/22, it does however fall into 172.16.0.0/16 as well as 0.0.0.0. It will use 172.16.0.0/16 path because it is the most specific of the two options

srg

LBC90805 wrote:

Well this is my dilemma. Don't both routes in the rouing table match each IP? The router would use 172.16.0.0/22 instead of 172.16.0.0/16 in both cases. First 172.16.0.0/16 is not using any subnetting what so ever, 172.16.0.0/22 is! Second I don't know why 172.16.0.0/16 would be used over 172.16.0.0/22, in every instance 172.16.0.0/22 remains the most specific route. `Am I missing something here?

Thanks for everyone's input thus far

172.16.0.0/22 is the most specifit route to THAT net.. the 172.16.0.0/22 net, which ranges from 172.16.0.0 to 172.16.3.255.. in other words, thats not a specific route to 172.16.4.3.

LBC90805

KRIPES, Brain Fart. That last little bit just sunk it into my thick skull for me.

Sorry, I've been very DULL these past odd days. Trying to manage to study with a Migraine just about everyday and some good and proper medicine, go ahead and imagine what I might be taking for pain because I probably `am; doctor prescribed of course!

Now if that was a /21 network then the 4.3 network would match that route?

gojericho0

bingo!

Markey

Hello I've been really struggling with this concept and I think I understand what SRG is saying;

What I'm confused about is:

172.16.0.0/22 implies the following subnets:

172.16.0.0 range 0.1 - 3.254 (3.255 = broadcast)
172.16.4.0 range 4.1 - 7.254 (7.255 = broadcast)
172.16.8.0 range 8.1 - 11.254 (11.255 = broadcast)
.
.
172.16.252.0 range 252.1 - 255.254 (255.255 = broadcast)

Question is why does the router not consider the other subnets as valid for this route and only the first range as specific for the route; As you said 172.16.4.3 falls into the second subnet range. Is it that the router only considers the zero subnet when choosing?

Help I'm drowning!

Paul Boz

The router doesn't consider it because it's not in the routing table. If the routing table had a route for 172.16.4.0 /22 it would use that route for the second address. In the absence of a more specific route, however, the router uses the 172.16.0.0 /16 route.

suffah

Yo Markey,

I see you are still struggling a bit with this concept from the other thread:

http://www.techexams.net/forums/viewtopic.php?p=235259#235259

Markey wrote:

172.16.0.0/22 implies the following subnets:

172.16.0.0 range 0.1 - 3.254 (3.255 = broadcast)
172.16.4.0 range 4.1 - 7.254 (7.255 = broadcast)
172.16.8.0 range 8.1 - 11.254 (11.255 = broadcast)
.
.
172.16.252.0 range 252.1 - 255.254 (255.255 = broadcast)

No, 172.160.0.0/22 is one specific subnet. It does not imply every possible subnet out there that fits under the /22 netmask.

For example, 172.160.4.0/22 is a different subnet.

The router table would need 172.160.4.0/22 if you wanted that subnet and so on.

Markey

yep I understand now. The part that was confusing me was the /22 in regards to subnetting. The router doesn't make it's decision based on all the subnets listed with that classful network (172.16.0.0) and mask. As you say 172.16.4.0/22 would match 172.16.4.1-172.16.7.254 range. 172.16.8.0/22 would match an address range of 172.16.8.1-172.16.11.254, 172.16.16.0/22 would match 172.16.16.1 - 172.16.19.254 etc. etc.... Thanks for the response.

suffah

Bingo, you got it!

By chance, did you learn subnetting via CBT nuggets? I can see how you could get confused because in his teaching examples he always listed the possible subnets for each subnet mask.

Markey

Hi Suffah thanks for your support. No I've been using the Odom books and the free Boson test environment that came with it.

Thanks again!