Problem using ip subnet zero

Answered Question

I am setting up a router for an MPLS migration and I gave my provider a list of /30 address. They assigned me the subnet address for my CE routers. I am getting bad mask when I try to add the ip address 192.168.1.40/30. I am running 12.4.11T on my 2811 ISR. Shouldn't this work? I have verify ip subnet-zero and ip classless are there? Is this a bug?

I have this problem too.
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Correct Answer by purohit_810 about 9 years 7 months ago

Hi,

There is not IP addressing rule to use /31.

We CANNOT USE /31 as a Subnet. at leaset 2 Bit are required for subnet. THAT IS THE RULE.

If you can count.... subnetting for host.

1111 1111.1111 1111.1111 1111.1111 1110 mean /31 corect..

do for host 2 to power 1 = 2 - 2 (For subnet and broadcast ID)= 0 (NO HOST REMAINS).

Regards,

Dharmesh Purohit

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Overall Rating: 3.5 (6 ratings)
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Richard Burts Tue, 02/20/2007 - 09:43

Wayne

With a /30 mask 192.168.1.40 is the subnet address and can not be configured as an interface address (and that is why the error is bad mask - not entirely intuitive but it is the /30 mask that makes it invalid - with a /28 mask that address would be quite valid). Within that /30 mask the host addresses would be .41 and .42. Try putting one of those on the interface and let us know what happens.

HTH

Rick

purohit_810 Tue, 02/20/2007 - 09:48

Rick is true.... Please try such a way.

Second, if you are trying .41 or .42 address and still it is giving same error. in that situation, please make sure...... what IP addres used at ISP end mean at PE router. choose different ip address than PE.

Please let us know, still problem persist.

Regards,

Dharmesh Purohit

Correct Answer
purohit_810 Tue, 02/20/2007 - 10:12

Hi,

There is not IP addressing rule to use /31.

We CANNOT USE /31 as a Subnet. at leaset 2 Bit are required for subnet. THAT IS THE RULE.

If you can count.... subnetting for host.

1111 1111.1111 1111.1111 1111.1111 1110 mean /31 corect..

do for host 2 to power 1 = 2 - 2 (For subnet and broadcast ID)= 0 (NO HOST REMAINS).

Regards,

Dharmesh Purohit

purohit_810 Tue, 02/20/2007 - 10:35

Hi,

It is usefull only in Point-to-point Link.

Second you are talking about core (CE) routing into root quetion.

Richard Burts Tue, 02/20/2007 - 11:14

Do not say:

"We CANNOT USE /31 as a Subnet. at leaset 2 Bit are required for subnet. THAT IS THE RULE" when it is in fact NOT the rule.

HTH

Rick

purohit_810 Tue, 02/20/2007 - 15:13

Hi Rick,

I am hounering your solutions.

What router accept that is not IP addressing rule.

Ip addressing has open standard rules.

Now, Router vendor either cisco or else that is regardless, what design. IOS and firmware accept the same mask. It has not concern with that.

IP address RULE are different and IOS/firmware accept commands that is different.

That is what i was telling.

Why we are giving ip subnet-zero command... to indicate IOS... we are going to break the IP adress rule. That is our requirement and please accept subnet mask what we are giving.

rtanner Tue, 02/20/2007 - 21:58

This situation is covered by the following RFC:

RFC 3021 - Using 31-Bit Prefixes on IPv4 Point-to-Point Links

purohit_810 Wed, 02/21/2007 - 08:01

Hi Rick,

Thanks... I will check out the same.

Thanks a Lot.

Regards,

Dharmesh Purohti

Richard Burts Thu, 02/22/2007 - 11:38

Wayne

I am glad that we were able to help you resolve your problem. Thanks for using the rating system. It makes the forum more useful when people can read about a problem and can know that they will also read a solution that resolved the problem.

I encourage you to continue your participation in the forum.

HTH

Rick

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