Find the Matching Route chapter 15 – Page 484 of Cisco’s ccent/ccna ICND1 book.

damien2008

Hi,

Am having some trouble understanding the explanation of ‘finding the matching route’ in chapter 15 – Page 484 of Cisco’s ccent/ccna ICND1 book.

Compare the packet’s destination to the ranges of addresses and find all the matching routes.
In cases were a particular destination IP address falls within the IP address range for multiple routes, pick the route with the longest prefix length.

The destination addresses are

· 172.16.1.1
· 172.16.1.2
· 172.16.2.2
· 172.16.4.3

Explanations from the book are below:

172.16.1.1 matches all 5 routes, but the host route for specific IP address 172.16.1.1 prefix length /32 has the longest prefix length

172.16.1.2 matches 4 of the routes (all except the host route for 172.16.1.1) but the route to 172.16.1.0 /24 has the longest prefix

172.16.2.2 matches the last 3 routes listed in R1’s routing table in the example with the route for 172.16.0.0 /22 having the longest prefix length

172.16.4.3 matches the last 2 routes listed in R1’s routing table in the example, with the route for 172.16.0.0 /16 having the longest prefix

Doesn’t 172.16.4.3 match the last 3 routes, therefore 172.16.0.0 /22 should be the matching route instead of 172.16.0.0 /16 as 172.16.0.0 /22 has the longest prefix ?

Is this a mistake in the book

: or am I wrong

:
If I am wrong can someone please expand on this....

Kind regards

Damien

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Comments

lordy

172.16.0.0/22 is 172.16.0.0 -> 172.16.3.255, so 172.16.4.3 does not match.

CodeBlox

As stated above, the route would only match the last two. And the /16 has the longer prefix length. 172.16.4.3 does not fall into the 172.16.0.0/22 network range

hiddenknight821

damien2008 wrote: »

Doesn’t 172.16.4.3 match the last 3 routes, therefore 172.16.0.0 /22 should be the matching route instead of 172.16.0.0 /16 as 172.16.0.0 /22 has the longest prefix ?

Is this a mistake in the book : or am I wrong :
If I am wrong can someone please expand on this....

I'm afraid to tell you that Lordy and Odom's book was right. I can see where you got confused. Even though 172.16.0.0/22 seems like it has a route for 172.16.4.3. What fooled you was the last 2 octets being zero. Don't confuse this 172.16.0.0/22 with 172.16.0.0/16.

Here's a rule of

you should keep in mind. If have two routes with the same "address" but different CIDR notation, then use that notation to aid you. You have to master the subnetting to see where the destination address fits in the routing table. What I did was to see which octets I can completely ignore.

In this example, I can only ignore the last octet. The first three is very important. Any destination address that go to the route 172.16.0.0/22 must have one of the addresses between 172.16.0.1 and 172.16.3.254. Did you see the bold text? This is why I said it is important to pay attention to the first three octets. Thus, 172.16.4.3 doesn't fit. I hope this helps.

ccnxjr

damien2008 wrote: »

Hi,

Doesn’t 172.16.4.3 match the last 3 routes, therefore 172.16.0.0 /22 should be the matching route instead of 172.16.0.0 /16 as 172.16.0.0 /22 has the longest prefix ?

Is this a mistake in the book : or am I wrong :
If I am wrong can someone please expand on this....

Kind regards

Damien

I think you get the idea, i'll try to restate it a little differently,

So, lets go back to binary (well, on the last two octets anyway)
expanding the ip address
172.16.4.3
172.16. 0000 0100 . 0000 0011

expanding the mask for route 172.16.0.0 /22
172.16. 1111 1100 . 0000 0000
This route will work for ranges
172.16.0.1 up to
172.16.3.254

expanding the mask for the other route 172.16.0.0/16
172.16.0.0
This route will work for ranges
172.16.0.1 up to
172.16.255.254

Don't feel discouraged, take it slow for this chapter, or complete the others and revisit that chapter again.
Subnetting (or related activities) takes practice and is a continuous process.

techie2012

I have a

:, since I was confused about this too. Since it was network 172.16.0.0/22 and that would be be addresses 172.16.0.1-172.16.3.255, the next subnet would be 172.16.4.0 and addresses being 172.16.4.1-172.16.7.255. Is it because RIP is a classful protocol that 172.0.0.0/22 cant summarize for the address 172.16.4.3 as well. For instance, would 172.16.4.3 be included with 172.16.0.0/22 if it was RIPv2?

lordy

That does NOT depend on the routing protocol involved. 172.16.4.3 never matches 172.16.0.0/22.

ccnxjr

I fear i may have confused you even more, sorry!
i wrote that BC (before coffee

).
I mis-used the word "mask" and made up my own soup of mask/network (thats kinda how i visualize it in my head....)

172.16.0.0/22 means that 22 bits (from left to right) have to match in order to fall within that network ie
172.16. 0000 0000 . 0000 0000

172.16.4.3 only matches 21 bits (from left to right), the 22nd bit is different
172.16. 0000 0100 . 0000 0011

172.16.4.3 would've matched 172.16.0.0/21

172.16.0.0/21 would include addresses from 172.16.0.1 through 172.16.7.254

damien2008

Thanks for all the comments and help, I understand a bit better now.
The thing I was confused with was because 172.16.0.0 /22 went up in 4's,

0
4
8
12
16
etc

The 172.16.4.3 is within the /22 range, but its not in the subnet listed in the router table (172.16.0.0 /22) if the router table had 172.16.4.0 /22 listed, then this would be the matching route....

Thanks