Question re: most appropriate route

drew2000

This one should be pretty easy, but it's not clicking for me. (Odom ICND1 pg. 484)

The following 2 routes are in a routing table: (172.16.0.0/16 is variably subnetted, 5 subnets, 4 masks) **I removed some that aren't pertinent to my question:

172.16.0.0/22
172.16.0.0/16

Odom says that the following IP address only matches the /16 route: 172.16.4.3.

I don't understand this because shouldn't the 172.16.4.0 subnet be a part of the /22 network?

It's possible I am tired and only halfway thinking this through...I'm usually pretty strong on subnetting but this one has thrown me off tonight. It must be the variable subnetting.

Drew

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Comments

jmc012

drew2000 wrote: »

This one should be pretty easy, but it's not clicking for me. (Odom ICND1 pg. 484)

The following 2 routes are in a routing table: (172.16.0.0/16 is variably subnetted, 5 subnets, 4 masks) **I removed some that aren't pertinent to my question:

172.16.0.0/22
172.16.0.0/16

Odom says that the following IP address only matches the /16 route: 172.16.4.3.

I don't understand this because shouldn't the 172.16.4.0 subnet be a part of the /22 network?

It's possible I am tired and only halfway thinking this through...I'm usually pretty strong on subnetting but this one has thrown me off tonight. It must be the variable subnetting.

Drew

That one confused me too and he doesn't really explain it, here is a post I found on the internet back when I was trying to figure it out. I saved it in my notes. I thought it was a good post.

Take a bit-wise approach to it.....
172.16.0.0/22 is equal to this:
172.16. 0000 0011. 1111 1111
So, if you only have the first two bits in the 3rd octet, the range you get is 172.16.0.0 to 172.16.3.255, mask 255.255.252.0.
172.16.4.3 would match the specific 172.16.4.0/22 network, but that is not an option. Hence, it only matches the /16 route. However, 172.16.2.2 falls within both 172.16.0.0/22 and 172.16.0.0/16.
Key to remember is that routers see all of these values as a series of 0's and 1's, so breaking it down to that level, you see what the router sees. Appy the same logic and you're golden. When I have to do this with more complicated masks and networks, I break out a pad and paper and write it all out. Sometimes it comes down to using the binary AND method, it's almost foolproof.

tech-airman

drew2000,

drew2000 wrote: »

This one should be pretty easy, but it's not clicking for me. (Odom ICND1 pg. 484)

The following 2 routes are in a routing table: (172.16.0.0/16 is variably subnetted, 5 subnets, 4 masks) **I removed some that aren't pertinent to my question:

172.16.0.0/22
172.16.0.0/16

What were the routes you omitted?

drew2000 wrote: »

Odom says that the following IP address only matches the /16 route: 172.16.4.3.

Odom is a CCIE so I'd imagine he'd be careful with what he wrote. Have you gone to http://www.ciscopress.com and updated your book's text with the errata?

drew2000 wrote: »

I don't understand this because shouldn't the 172.16.4.0 subnet be a part of the /22 network?

I don't understand either.

drew2000 wrote: »

It's possible I am tired and only halfway thinking this through...I'm usually pretty strong on subnetting but this one has thrown me off tonight. It must be the variable subnetting.

Drew

Which routing protocol(s) is/are in use?

mattrgee

The books asks this question:

What route would match the following IP address:

172.16.1.1
172.16.1.2
172.16.2.2
172.16.4.3

The available routes are:

172.16.1.1/32
172.16.1.0/24
172.16.0.0/22
172.16.0.0/16

The first 3 are straightforward. The answer for 172.16.4.3 is given as 172.16.0.0/16 because it has the longest prefix. However, As the Op says, 172.16.0.0/22 is a better match. The routing protocol is RIP.

The errata doesn't list this as an error. Perhaps auto summary is enabled in the example?

mattrgee

The book is right.

If you look at the network ranges that the routes are matching only 172.16.0.0/16 matches the 172.16.4.3 address.

i.e.

Route: 172.16.0.0 /22 covers the 172.16.0.0 - 172.16.3.255 network, 172.16.4.3 is outside of this range and is therefore matched by the 172.16.0.0/16 route as it is more specific than the default route.

drew2000

Thanks for all the replies. I never doubted the book (although I checked the errata this morning).

I guess I must have slipped on my subnetting skills a bit. I'll work through this one again.

Drew

meadIT

The way I figure out address ranges is to convert the CIDR notation to decimal. /22 is a 255.255.252.0 mask, so your subnets are going to be in blocks of 4 (256-252 = 4). Your networks are going to be:

172.16.0.0
172.16.4.0
172.16.8.0
.
.
.

drew2000

meadIT,
I was just about to post my math, and it is exactly the same as yours.

So, given your answer, doesn't 172.16.4.3 fall within the 172.16.4.0 network?

The subnet number would be 172.16.4.0
The broadcast address would be 172.16.7.255
The valid host range would be 172.16.4.1 ->172.16.7.254

Maybe I'm missing something dumb here.

Drew

mattrgee

Keep in mind the route listed is for the network listed and not all networks that can be derived from the mask.

So the route 172.16.0.0/22 is for the network 172.16.0.0 - 172.16.3.255, not networks 172.16.4.0 - 172.16.7.255, 172.16.8.0 - 172.16.11.255 etc etc Hence why the route doesn't match.

meadIT

drew2000 wrote: »

meadIT,
I was just about to post my math, and it is exactly the same as yours.

So, given your answer, doesn't 172.16.4.3 fall within the 172.16.4.0 network?

It does fall in the 172.16.4.0 network, but that wasn't one of your available networks:

The available routes are:

172.16.1.1/32
172.16.1.0/24
172.16.0.0/22
172.16.0.0/16

drew2000

Ahhhhhhhh click. That makes sense. The 0.0 threw me. I was thinking about that in the "all possible networks sense," not the "specific network of 172.16.0.0 sense".

Because 172.16.4.3 is NOT part of the specific network 172.16.0.0/22, the answer can't be right.

Thanks for all of the brainpower. This one may cause me to struggle on the test.

Drew

aordal

Awesome post. When I read that chapter last week I was confused and I reread the question over and over and couldn't figure it out. Makes sense now.