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Simple question on Subnetting

rkunta
Level 1
Level 1

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 :-)

1 Accepted Solution

Accepted Solutions

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.

View solution in original post

8 Replies 8

nambi_gct
Level 1
Level 1

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.

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.

Thank you Mahmoud, that really helps....

Regards,

Ramchander.

Hi Ramchander,

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

BR,

Mohammed Mahmoud.

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!)

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.

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.

Thank you Nambi....

Regards,

Ramchander

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