Simple question on Subnetting

Answered Question
Sep 3rd, 2007
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Greetings!!


If I have a /30 subnet, say x.x.x.0/30, I will use .1 & .2 and cannot use .0 since it is subnet and .3 cannot be used since it is a broadcast ip address.


But, instead, if I further subnet the /30 to /32, I will get four ip addresses, .1, .2,.3,.4/32; can I use all of them for four different loopbacks on four different L3 interfaces?


Thanks in advace :-)

Correct Answer by mohammedmahmoud about 9 years 9 months ago

Hi,


I think that Arivudainambi's answer covers your question, just to elaborate more, we can also use a /31 prefixes, in order to conserve IP address space on the Internet, a 31-bit prefix length (255.255.255.254) allows the use of only two IP addresses on a point-to-point link. Previously, customers had to use four IP addresses or unnumbered interfaces for point-to-point links.


IP directed broadcast not allowed with 31 bit mask (when entering a /31 address ip directed broadcast will be immediately disabled).


NOTE no ip directed-broadcast is the default on modern IOS and thus it might not appear in the configuration.


When entered on a non point-to-point interfaces the following error occurs: % Warning: use /31 mask on non point-to-point interface cautiously.


http://cisco.com/en/US/products/ps6350/products_configuration_guide_chapter09186a0080455093.html


HTH,

Mohammed Mahmoud.

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nambi_gct Mon, 09/03/2007 - 03:55
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Yes.It is valid.With /32 mask, logically broadcast address,subnet address and the host address are the same./32 is the recommended mask for loopback interfaces.

Correct Answer
mohammedmahmoud Mon, 09/03/2007 - 04:12
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Hi,


I think that Arivudainambi's answer covers your question, just to elaborate more, we can also use a /31 prefixes, in order to conserve IP address space on the Internet, a 31-bit prefix length (255.255.255.254) allows the use of only two IP addresses on a point-to-point link. Previously, customers had to use four IP addresses or unnumbered interfaces for point-to-point links.


IP directed broadcast not allowed with 31 bit mask (when entering a /31 address ip directed broadcast will be immediately disabled).


NOTE no ip directed-broadcast is the default on modern IOS and thus it might not appear in the configuration.


When entered on a non point-to-point interfaces the following error occurs: % Warning: use /31 mask on non point-to-point interface cautiously.


http://cisco.com/en/US/products/ps6350/products_configuration_guide_chapter09186a0080455093.html


HTH,

Mohammed Mahmoud.

rkunta Mon, 09/03/2007 - 04:19
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Thank you Mahmoud, that really helps....


Regards,

Ramchander.

mohammedmahmoud Mon, 09/03/2007 - 04:32
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Hi Ramchander,


You are very welcomed, and thank you for using the rating system.


BR,

Mohammed Mahmoud.

rkunta Mon, 09/03/2007 - 04:39
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You are welcome Mahmoud, one more quick question. In case of /31, how do I address the subnet? There are only two ip addresses possible here, but how do I address the subnet? (or will there be no necessity of using subet? I am not able to comprehend!)

mohammedmahmoud Mon, 09/03/2007 - 06:06
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Hi Ramchander,


It is just used for point-to-point links, where only one end is trying to communicate with the other end.


HTH,

Mohammed Mahmoud.

chintan-shah Tue, 07/29/2008 - 01:34
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Hi Mohammed,


I am planning to use /31 and I tested that it works fine. But I wonder in one my ACL what should be wildcard i use ? 0.0.0.1 for /31 or soemthing else ?


chintan.

rkunta Mon, 09/03/2007 - 04:18
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Thank you Nambi....

Regards,

Ramchander

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