VLSM for trunking

Unanswered Question
Jun 20th, 2008
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I'm trying to break up a /24 subnet for one of my sites in order to create VLANs.

I want to break up 192.168.100.0/24 like this:


192.168.100.1/26 (for 62 hosts)

192.168.100.65/25 (for 126 hosts)


I know there are portions of the subnet left over but I don't need them currently. When trying to configure the router (Cisco 2651XM) for subinterfaces I get the error:


Router(config)#int f0/0.10

Router(config-subif)#encap dot1q 10

Router(config-subif)#ip address 192.168.100.1 255.255.255.192


Router(config)#int f0/0.11

Router(config-subif)#encap dot1q 11

Router(config-subif)#ip address 192.168.100.65 255.255.255.128

% 192.168.100.0 overlaps with FastEthernet0/0.10


If I'm using VLSM then the /26 subnet I placed on f0/0.10 would range from 192.168.100.0-63 and would leave the 192.168.100.65 address available to put in a different subnet. What am I missing?

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Jon Marshall Fri, 06/20/2008 - 07:00
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You can't subnet it the way you have. Change it to


192.168.100.0/25

192.168.100.128/26


192.168.100.0/25 will give you


192.168.100.1 -> 192.168.100.126 with .127 as the broadcast address.


192.168.100.128/26 will give you


192.168.100.129 -> 192.168.100.190 with .191 as the broadcast address.


Edit - have at a look at this thread for why your original subnetting wouldn't work.


Jon

Jon Marshall Fri, 06/20/2008 - 07:47
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michael.leblanc Fri, 06/20/2008 - 07:53
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Quinton:


Your statement:


"If I'm using VLSM then the /26 subnet I placed on f0/0.10 would range from 192.168.100.0-63 and would leave the 192.168.100.65 address available to put in a different subnet. What am I missing?"


... demonstrates that you have carefully considered 192.168.100.1/26, but then you transition to assumption.


You need to consider 192.168.100.65/25, and the address space that represents.


You have focussed on "65", whereas your focus should be on the "/25" mask.


As Jon suggested, its best that you reconsider it, and arrive at the understanding yourself.


raj_ctech1103 Mon, 06/23/2008 - 00:26
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Hi,

As your subnet /25 overlaps the subnet /26.So better will be if u use

192.168.100.0-63 for f0/0.10

&

192.168.100.128-255 for f0/0.11

pls let me know the status.


regards,

Raj

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