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 Post subject: Bad Mask
PostPosted: Fri Oct 15, 2010 9:46 am 
I am trying to input a loopback address of 10.10.1.0 255.255.255.0 I get a bad mask error.

I believe the ip subnet-zero is in by default as I can input the no ip subnet-zero.

I need this for a lab submission, can anyone assist.

Thanks




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 Post subject: Re: Bad Mask
PostPosted: Fri Oct 15, 2010 8:52 pm 
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Joined: Fri Mar 05, 2010 11:33 am
Posts: 1494
Location: Australia
The message is absolutely correct - 255.255.255.0 IS INDEED A BAD MASK for 10.10.1.0
AND it has absolutely nothing to do with the zero subnet.

So I guess you'd like to know why so?

The reason is very simple - 10.10.1.0 is the NETWORK address for the 10.10.1.0/24 subnet. 10.10.1.255 is the BROADCAST address for the 10.10.1.0/24 subnet - and if you tried to use that - you would get the same error.

But since this is on the loopback interface, it should make no difference to you if you use 10.10.1.1 or 10.10.1.254 or anything in between. That's IF you want to stick with that 24 bit mask.

Now when it comes to loopback addresses, why allocate a whole subnet? - there will NEVER be any other devices to communicate with on that subnet.*
So what most engineers do is assign a 32 bit mask to the loopback e.g.

interface lo0
ip address 10.10.1.0 255.255.255.255


HTH

Note:
*It can be useful to allocate subnets to the loopback if you want to build up a few extra networks to boost the size of your routing table.

Note2:
The ip subnet-zero would be required if you wanted to use 10.0.0.1 255.255.255.0 because this subnet (10.0.0.0/24) is a zero subnet on the 10.0.0.0 network.

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GNS3 WorkBench-a VMware image of Ubuntu with GNS3 and VPCS installed and a collection of exercises/labs


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 Post subject: Re: Bad Mask
PostPosted: Sat Oct 16, 2010 9:00 am 
rednectar wrote:
The message is absolutely correct - 255.255.255.0 IS INDEED A BAD MASK for 10.10.1.0
AND it has absolutely nothing to do with the zero subnet.

So I guess you'd like to know why so?

The reason is very simple - 10.10.1.0 is the NETWORK address for the 10.10.1.0/24 subnet. 10.10.1.255 is the BROADCAST address for the 10.10.1.0/24 subnet - and if you tried to use that - you would get the same error.

But since this is on the loopback interface, it should make no difference to you if you use 10.10.1.1 or 10.10.1.254 or anything in between. That's IF you want to stick with that 24 bit mask.

Now when it comes to loopback addresses, why allocate a whole subnet? - there will NEVER be any other devices to communicate with on that subnet.*
So what most engineers do is assign a 32 bit mask to the loopback e.g.

interface lo0
ip address 10.10.1.0 255.255.255.255


HTH

Note:
*It can be useful to allocate subnets to the loopback if you want to build up a few extra networks to boost the size of your routing table.

Note2:
The ip subnet-zero would be required if you wanted to use 10.0.0.1 255.255.255.0 because this subnet (10.0.0.0/24) is a zero subnet on the 10.0.0.0 network.


Your Note * is correct, my instructor has requested that we use these on the loopback so that we can advertise the network... he put down to address as the network id with a /24 mask. I have already add with the 10.10.1.1 address and will advertise the 10.10.1.0 network. Thanks for the reply.


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 Post subject: Re: Bad Mask
PostPosted: Tue Jan 18, 2011 10:47 am 
still the same issue of bad mask ? is the solution


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 Post subject: Re: Bad Mask
PostPosted: Tue Jun 25, 2013 1:31 pm 
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Joined: Tue Jun 25, 2013 1:29 pm
Posts: 1
Please try to give the IP Address to a loopback interface




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