Question about ip subnet zero

steve2012 · May 2012

I am a little confused about ip subnet zero. I know the basics, that if RIPv2 is in play it is permitted, and that the default on Cisco routers today is default on. But I am having difficulty wrapping my head around the concept. I have read up on the Internet as well as Odems book and was wondering is someone can maybe explain it in a simpler form.

I am obviously not quite there yet and would appreciate a quick run down from anothers perspective.

Thank you

Steve

georgemc · May 2012

This link points to just about the clearest explanation that I've seen.

Subnet Zero and the All-Ones Subnet* [IP Addressing Services] - Cisco Systems

steve2012 · May 2012

Thank you,
Hopefully it will sink in for tomorrows ICND1

CodeBlox · June 2012

Yikes, you don't want to really be unsure about objectives the night before these exams!

Carl_S_901 · June 2012

Steve,

If you are familiar with the host and broadcast address for a subnet being unusable for hosts (which you should be) then you should get it because the concept is the same. Let me give you and example.

Let's say we have 192.168.1.0/24 and we are subnetting it into /27 networks. We would have the following networks:

192.168.1.0
192.168.1.32
192.168.1.64
192.168.1.96
192.168.1.128
192.168.1.160
192.168.1.192
192.168.1.224

or a total of 8 usable subnets when using the default of allowing subnet zero.

Using no ip subnet zero on the router means that we cannot use the all 0's are all 1's subnets. All 1s subnet may be referred as the broadcast subnet. Under that restriction, our available subnets would be:

192.168.1.0 (cannot use)
192.168.1.32
192.168.1.64
192.168.1.96
192.168.1.128
192.168.1.160
192.168.1.192
192.168.1.224 (cannot use)

or a total of 6 usable subnets when we apply the no ip subnet zero command to the router.

The original reasoning behind this is that 192.168.1.0 would be the network address for both 192.168.1.0/24 and 192.168.1.0/27. Also the broadcast address of 192.168.1.255 would be the the same for 192.168.1.0/24 and 192.168.1.224/27. In theory there would be no way to distinguish between the 2 network addresses or the two broadcast addresses. Modern routers can handle this fine though.

The crappy thing about all of this is that the concept of not using the all 0s or all 1s subnet was obsoleted in RFC 1878 which was published in......drumroll please...... 1995. So no one in the real world has been avoiding the use of the all 0s and all 1s subnets for many years. I'm not sure why they still teach this except for theory/foundation. Imagine the amount of wasted public IP address space if we followed this on the Internet? We would have run out of public addresses a very long time ago.

Hope this helps.

Thanks

Carl S.

Check out my personal certification journey blog
Carl's Certification Journey | The road to getting certified can often be bumpy

Mstavridis · June 2012

Its taught cause you can still run into the issue on dated hardware that connects to your current hardware.

Carl_S_901 · June 2012

Yes, technically it was not obsoleted "decades" ago so I edited my post to reflect that. I'm guessing that a piece of gear would have to be more than 15 years old for this to be an issue?

Mstavridis · June 2012

Well yes and alot, and I mean ALOT haha will be around the 20 years old mark. No one throws this equipment away, what happens is they just but a brand new piece of equipment next to it as it would be to much work to fully decommission it.

Humans = Lazy, haha

fadhil · June 2012

please remember that for cisco router all subnet are using regardless of subnet zero, because it can handle .

Mstavridis · June 2012

fadhil wrote: »

please remember that for cisco router all subnet are using regardless of subnet zero, because it can handle .

Unless it is 15+ years old

networker050184 · June 2012

If you are working with 15+ year old gear its time to find a new job!

steve2012 · June 2012

Thank you for all the responses, much appreciated!!

Steve

Carl_S_901 · June 2012

So how did you do on the test Steve? Lets here the good news.

steve2012 · June 2012

Carl_S_901 wrote: »

So how did you do on the test Steve? Lets here the good news.

Made it! Very pleased.

Thnx

JustFred · June 2012

Congratulations and good luck on the next

Question about ip subnet zero

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