VLSM for trunking

qbakies11 · ‎06-20-2008

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?

Jon Marshall · ‎06-20-2008

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 · ‎06-20-2008

Sorry, having a bit of a moment :-)

Here's the link to the thread.

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Network%20Infrastructure&topic=WAN%2C%20Routing%20and%20Switching&topicID=.ee71a06&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.2cc0c6bf/2#selected_message

Jon

michael.leblanc · ‎06-20-2008

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 · ‎06-23-2008

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