Subnet Zero

Okay this concept is confusing to me. Subnet zero is 0.0.0.0? Apparently for 12.0 or higher IOS, it's automatically enabled which likely means I've been using it for years and didn't know it. I'm pretty sure I don't have a solid understanding of subnet-zero. How can knowledge of subnet zero make any sense out of the following question on the practice exam?

Okay, so at Company X we need to subnet 192.168.164.0 and divide subnets for teams. As network admin you will configure subnet-zero and use RIPv2 as routing protocol. You need to conserve unused address space for future growth.

There are 3 groups:
Group 1 has 11 hosts
Group 2 has 16 hosts
Group 3 has 6 hosts
(Group 4 = connection to WAN)

Correct Answer:
1 = 192.168.164.166 /28
2 = 192.168.164.149 /27
3 = 192.168.164.255 /26
4 = 192.168.164.189 /30

HUH???

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Comments

blackninja

godofthunder9010 wrote: »

Okay this concept is confusing to me. Subnet zero is 0.0.0.0? Apparently for 12.0 or higher IOS, it's automatically enabled which likely means I've been using it for years and didn't know it. I'm pretty sure I don't have a solid understanding of subnet-zero. How can knowledge of subnet zero make any sense out of the following question on the practice exam?

Okay, so at Company X we need to subnet 192.168.164.0 and divide subnets for teams. As network admin you will configure subnet-zero and use RIPv2 as routing protocol. You need to conserve unused address space for future growth.

There are 3 groups:
Group 1 has 11 hosts
Group 2 has 16 hosts
Group 3 has 6 hosts
(Group 4 = connection to WAN)

Correct Answer:
1 = 192.168.164.166 /28
2 = 192.168.164.149 /27
3 = 192.168.164.255 /26
4 = 192.168.164.189 /30

HUH???

ip subnet zero

It's a command on a cisco router and from IOS 12 and later it is on there by default.

It just means you can use the first and last subnets when you break a network into subnet i.e.

without subnet zero:

You couldn't use the first and last subnet
192.168.0.0 /24

192.168.0.0 NOT USED
192.168.0.16
192.168.0.32
.
.
192.168.0.224
192.168.0.240 NOT USED

with ip subnet zero you can use the first and last subnets:

192.168.0.0 NOW USED
192.168.0.16
192.168.0.32
.
.
192.168.0.224
192.168.0.240 NOW USED

Make sense?

Forsaken_GA

As black ninja said, as part of the holdover from classful subnetting, once upon a time, you couldn't use the very first subnet in an allocation (called subnet 0) because it contained the network identifier for the entire allocation. Nor could you use the last subnet because it contained the broadcast address for the entire allocation.

This was what I was taught when I began the CCNA as well, Cisco's curriculum occasionally lies to you about some concepts to make them easy to learn, then they toss the curveball at you when you advance your studies.

That question is basically just a VLSM question, and not a very well worded one at that.

godofthunder9010

Okay that's making a lot more sense now. Apparently I've not ever had to wrestle with subnet-zero disabled. Lucky me! Sounds pretty annoying!

Okay, as a VLSM question, the answer still makes no sense to me. It looks thrown together willy-nilly. Normally, I'd start with the biggest subnet first and work my way down to the smallest if I'm trying to conserve space. It's a lousy example of course. If there were 300 users in one group and 5 in another ... that sort of thing, then the example fits a lot better.

Still, if it's on the practice exam, then there is a EXTREMELY HIGH likelihood that I'll see something a lot like it on the real CCNA test.

Forsaken_GA

I'd have to see the entire question to be able to determine if those answers are correct or not. The answers for 1, 2 and 4 seem reasonable enough, the answer for is kind of odd, allocating a /26 for 6 users is not a good way to conserve IP space, that'll waste 24 IP's, so unless group 3 is expecting growth, that doesn't seem quite right

godofthunder9010

I'm going to operate under the assumption that they simply screwed up that question, and that's that. There's a couple of broken questions in the 400-odd questions.

Forsaken_GA

Yeah, they probably meant the mask for the third group to be a /29 instead of a /26, every test prep package has little mistakes here and there

godofthunder9010

Yeah if you make that correction and change the last octet, then going largest group to smallest goes from this:

192.168.164.149 /27
192.168.164.166 /28
192.168.164.255 /26
192.168.164.189 /30

192.168.164.149 /27 (192.168.164.128 - 159)
192.168.164.166 /28 (192.168.164.160 - 175)
192.168.164.177 /29 (192.168.164.176 - 183)
192.168.164.189 /30 (192.168.164.184 - 191)

That scheme would make sense and it would also maximize IP conservation. I guess you get value out of even the questions they screwed up on but figuring out what the right "correct answer" should have been.