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subnetting

when subnetting , say a 240 mask, you create the 16, 32,48 networks etc, what happens from 0 to 16, is this useable and if not why not ?

4 REPLIES
Silver

Re: subnetting

In older IOS version the subnet zero was not usable.

When CIDR was introduced to allow sub-subnetting the command "ip subnet-zero" was introduced to configure the use of the zero subnet.

The answer is yes, you can use the subnet 192.168.1.0/28 for instance.

HTH

Leon

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New Member

Re: subnetting

ok, thankyou, can you give me an example of this, so your saying if you issue the command ip subnet zero, you can make use of the 0 to whatever the start of the first subnet is ?

thanks

Cisco Employee

Re: subnetting

Correct. If "ip subnet-zero" is not configured (it is configured by default) you will get an error message as follow:

R1(config)#int e0/0

R1(config-if)#ip address 192.168.1.1 255.255.255.240

Bad mask /28 for address 192.168.1.1

And now after configuring "ip subnet-zero":

R1(config-if)#ip subnet-zero

R1(config)#int e0/0

R1(config-if)#ip address 192.168.1.1 255.255.255.240

R1(config-if)#

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
New Member

Re: subnetting

Nevermind,

Leon answered already.

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