2nd Example on Pg 330 6th Ed.

[typesh]

Hey I just had a quick question about the 2nd example on Page 330. I understand how to figure out subnets/host IDs, but not sure what I am missing here...

The question is as follows:

Based on the output of the next routing table, which interface will a packet with a destination address of 10.10.10.14 be forwarded from?

Lab_A#sh ip route
[output cut]
Gateway of last resort is not set
C 10.10.10.16/28 is directly connected to FastEthernet0/0
C 10.10.10.8/29 is directly connected to FastEthernet0/1
C 10.10.10.4/30 is directly connected to FastEthernet0/2
C 10.10.10.0/30 is directly connected to Serial0/0

The answer is: 10.10.10.14 would be a host in the 10.10.10.8/29 subnet connected to FastEthernet0/1, which I understand.

But 10.10.10.14 is also a host on 10.10.10.4/30 and 10.10.10.0/30.

Why would it only be forwarded from FastEthernet0/1... and not from FastEthernet0/2 and Serial0/0?

Thanks...

[lammle]

No, host .14 is not in any of those other subnets. Look at it again. My example is correct. Redo the subnets and let me know what you come up with.
Todd

[typesh]

Oh my god... I cannot believe I overlooked this...!!

For some reason did the usual 256 - 252 = 4, and continued counting by block sizes until I found .14 as a host! I think I got carried away..

10.10.10.4/30 only contains 10.10.10.5 and 10.10.10.6 and broadcast 10.10.10.7

and

10.10.10.0/30 only contains 10.10.10.1 and 10.10.10.2 and broadcast 10.10.10.3

10.10.10.14 would only occur on the 10.10.10.12/30 network!


Thank you very much!!
:D

[lammle]

Look how much more you learned now!
Mistakes are our friends. :p
Great job with sticking with this!
Cheers!
Todd Lammle

[Fuzz]

While this example only has one clear option, there may be times in which a packets' destination has more than one route matched in the routing table. Let's say that the third option was in fact 10.10.10.12/30. Which route would be chosen now?

It would be the 10.10.10.12/30 route, as it is a more specific route, i.e. it has the largest subnet mask.