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sh ip route question

Posted on 2010-08-30
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What does this mean?

 172.18.0.0/16 is variably subnetted, 2 subnets, 2 masks

I have 5 routers, 2 in particular are connected together via ethernet cable.  IP on R1 is 172.18.0.1.  IP on R2 is 172.18.0.2.  They are both using 255.255.0.0 subnet.  

How is this 'variably subnetted', and how are there 2 subnets and 2 masks?  I could see the 'L' interface being seen as subnetted, but not the 'C' interface since it is the default mask of 255.255.0.0.

Here is my full sh ip route:
R     172.15.0.0/16 [120/2] via 172.18.0.1, 00:00:06, FastEthernet0/0
R     172.16.0.0/16 [120/2] via 172.18.0.1, 00:00:06, FastEthernet0/0
R     172.17.0.0/16 [120/2] via 172.18.0.1, 00:00:06, FastEthernet0/0
      172.18.0.0/16 is variably subnetted, 2 subnets, 2 masks
C        172.18.0.0/16 is directly connected, FastEthernet0/0
L        172.18.0.2/32 is directly connected, FastEthernet0/0
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Question by:B1izzard
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Wissam earned 1000 total points
ID: 33564476
Hello,

Variably subnetted is due to the fact that one of them has the default mask of 255.255.0.0 (the /16)
and the second has 255.255.255.255 (the /32).
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Author Comment

by:B1izzard
ID: 33564494
So variably subnetted is not the same as VLSM?  Any idea why they would consider a class B 255.255.0.0 subnetted?
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Assisted Solution

by:Jimmy Larsson, CISSP, CEH
Jimmy Larsson, CISSP, CEH earned 1000 total points
ID: 33564503
It IS subnettet because you have a small part of it with a /32-mask. If you remove that loopback it will not say that it is variable subnetted anymore.

/Kvistofta
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Author Comment

by:B1izzard
ID: 33564531
I understand why the /32 would be considered subnetted, just not the /16.  I changed my subnet mask to /17 and now it looks like this:
      172.18.0.0/16 is variably subnetted, 2 subnets, 2 masks
C        172.18.0.0/17 is directly connected, FastEthernet0/1
L        172.18.0.1/32 is directly connected, FastEthernet0/1

Now this makes more sense, as there are now in my opinion 2 subnets, the /17 and /32.  But I guess if Cisco says 255.255.0.0 is a subnet, then it's a subnet.
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Expert Comment

by:Wissam
ID: 33564593
It simply identifies the /16 subnet, to be subnetted variably to a /17 (255.255.128.0) and to another /32 (255.255.255.255) networks.
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