Solved: Simple question on Subnetting

rkunta · ‎09-03-2007

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 :-)

mohammedmahmoud · ‎09-03-2007

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

nambi_gct · ‎09-03-2007

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.

mohammedmahmoud · ‎09-03-2007

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 · ‎09-03-2007

Thank you Mahmoud, that really helps....

Regards,

Ramchander.

mohammedmahmoud · ‎09-03-2007

Hi Ramchander,

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

BR,

Mohammed Mahmoud.

rkunta · ‎09-03-2007

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 · ‎09-03-2007

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 · ‎07-29-2008

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 · ‎09-03-2007

Thank you Nambi....

Regards,

Ramchander