Why does Cisco require IP subnet-zero command?

Raymond Mason

I was studying for my exam and came across a part in Todd's book that says you need to use the ip subnet-zero command in order to use the first and last subnet in your design. Why?

I mean why wouldn't the first and last subnet be available automatically? Its like you buy a house but you need to enter a secret command to get two additional rooms to your house. I just don't completely understand this logic.

koz24

ip subnet-zero is now the default in all modern IOS.

Raymond Mason

Thank you for your answer. Was it that old machines couldn't process that many subnets or some protocol was unable to handle those certain subnets?

koz24

Raymond Mason wrote: »

Thank you for your answer. Was it that old machines couldn't process that many subnets or some protocol was unable to handle those certain subnets?

Here's a good explanation from Cisco:

Using subnet zero for addressing was discouraged because of the confusion inherent in having a network and a subnet with indistinguishable addresses.With reference to our example above, consider the IP address 172.16.1.10. If you calculate the subnet address corresponding to this IP address, the answer you arrive at is subnet 172.16.0.0 (subnet zero). Note that this subnet address is identical to network address 172.16.0.0, which was subnetted in the first place, so whenever you perform subnetting, you get a network and a subnet (subnet zero) with indistinguishable addresses. This was formerly a source of great confusion.


Prior to Cisco IOS® Software Release 12.0, Cisco routers, by default, did not allow an IP address belonging to subnet zero to be configured on an interface. However, if a network engineer working with a Cisco IOS software release older than 12.0 finds it safe to use subnet zero, the ip subnet-zero command in the global configuration mode can be used to overcome this restriction. As of Cisco IOS Software Release 12.0, Cisco routers now have ip subnet-zero enabled by default, but if the network engineer feels that it is unsafe to use subnet zero, the no ip subnet-zero command can be used to restrict the use of subnet zero addresses.

Subnet Zero and the All-Ones Subnet - Cisco

theodoxa

Basically, it was assumed that it would create confusion. Personally, I think that excuse is ridiculous and if anything not allowing the zero subnet is confusing, but...that is the reason.

It is my understanding that ip subnet-zero has been assumed on the CCNA since at least 2007. That is, unless the question specifies that "no ip subnet-zero" has been configured, you should assume that the IOS 12/15 default (allows first and last subnets). If a question says that "no ip subnet-zero" has been configured (the IOS 10/11 default), then you can't use the first or last subnet.

At one time, the exams assumed the opposite. I remember when I took the NetAcademy classes (2001) being told by the instructor that we should assume "no ip subnet-zero" unless otherwise specified.

ebohlman

The reason was that purely classful routing protocols like RIP-1 and IGRP don't send subnet masks in their route advertisements, so there was no way a router could distinguish between a summary route for an entire classful network and a specific route for its first subnet.

Priston

http://www.ietf.org/rfc/rfc950.txt

https://learningnetwork.cisco.com/thread/32136