ip subnet zero

The "ip subnet zero" command sorts of confuse me. I am looking at some practice questions in the ICND2 book and just completely lost after I found the answer to the question. I thought zero subnets were subnets that started off with .0 and broadcast address for that subnet was .255.

So for example, I thought 10.1.1.1 255.255.255.0 would be a zero subnet. Can someone help me understand this a little better? I selected option A for my answer, but the correct answer is C.

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Comments

pham0329

I'm going to take a guess, as I'm not 100% sure since the subnet zero concept is pretty much legacy.

If you take a look at Answer C, the subnet mask is 255.254.0.0, which means that the block size is 2. This gives us a range of

10.0.0.0 - 10.1.255.255
10.2.0.0 - blah

The ip 10.1.2.2 falls in the 10.0.0.0 subnet, hence, it is in subnet 0

altjx

pham0329 wrote: »

I'm going to take a guess, as I'm not 100% sure since the subnet zero concept is pretty much legacy.

If you take a look at Answer C, the subnet mask is 255.254.0.0, which means that the block size is 2. This gives us a range of

10.0.0.0 - 10.1.255.255
10.2.0.0 - blah

The ip 10.1.2.2 falls in the 10.0.0.0 subnet, hence, it is in subnet 0

Ah, gotcha. That clears up everything. Thanks!!

SharkDiver

First off, pham0329 is correct, but I'm going to try to clarify.

"no ip subnet zero" means that the first and last subnets are not allowed.

Looking at answer "a", which is the easiest to explain:
We are subnetting the 10.0.0.0 network by stealing 16 bits.
This will create 65,536 subnetworks of 256 IP addresses, of which the first IP is the network and the last IP is the broadcast.
So, we will have 65,536 subnets of 254 hosts.
Because of the "no ip subnet zero", we cannot use the first subnet or the last subnet.
The first subnet ranges from 10.0.0.0 to 10.0.0.255
The last one ranges from 10.255.255.0 to 10.255.255.255
Answer "a" (10.1.1.1) doesn't fall in either of these ranges.

Looking at answer "b":
We are subnetting the 10.0.0.0 network by stealing 17 bits.
This will create 131,072 subnetworks of 128 IP addresses, of which the first IP is the network and the last IP is the broadcast.
So, we will have 131,072 subnets of 126 hosts.
Because of the "no ip subnet zero", we cannot use the first subnet or the last subnet.
The first subnet ranges from 10.0.0.0 to 10.0.0.127
The last one ranges from 10.255.255.128 to 10.255.255.255
Answer "b" (10.0.0.129) doesn't fall in either of these ranges.

Looking at answer "d":
We are subnetting the 10.0.0.0 network by stealing 22 bits.
This will create 4,194,304 subnetworks of 4 IP addresses, of which the first IP is the network and the last IP is the broadcast.
So, we will have 4,194,304 subnets of 2 hosts.
Because of the "no ip subnet zero", we cannot use the first subnet or the last subnet.
The first subnet ranges from 10.0.0.0 to 10.0.0.3
The last one ranges from 10.255.255.252 to 10.255.255.255
Answer "d" (10.0.0.5) doesn't fall in either of these ranges.

Lastly, looking at answer "c":
We are subnetting the 10.0.0.0 network by stealing just 7 bits.
This will create 128 subnetworks of 131,072 IP addresses, of which the first IP is the network and the last IP is the broadcast.
So, we will have 128 subnets of 131,070 hosts.
Because of the "no ip subnet zero", we cannot use the first subnet or the last subnet.
The first subnet ranges from 10.0.0.0 to 10.1.255.255
The last one ranges from 10.254.0.0 to 10.255.255.255
Answer "c" (10.1.2.2) falls in the first subnet, and therefore will not be accepted as an IP address for an interface.

I hope that helps in some way.

altjx

SharkDiver wrote: »

First off, pham0329 is correct, but I'm going to try to clarify.

"no ip subnet zero" means that the first and last subnets are not allowed.

Looking at answer "a", which is the easiest to explain:
We are subnetting the 10.0.0.0 network by stealing 16 bits.
This will create 65,536 subnetworks of 256 IP addresses, of which the first IP is the network and the last IP is the broadcast.
So, we will have 65,536 subnets of 254 hosts.
Because of the "no ip subnet zero", we cannot use the first subnet or the last subnet.
The first subnet ranges from 10.0.0.0 to 10.0.0.255
The last one ranges from 10.255.255.0 to 10.255.255.255
Answer "a" (10.1.1.1) doesn't fall in either of these ranges.

Looking at answer "b":
We are subnetting the 10.0.0.0 network by stealing 17 bits.
This will create 131,072 subnetworks of 128 IP addresses, of which the first IP is the network and the last IP is the broadcast.
So, we will have 131,072 subnets of 126 hosts.
Because of the "no ip subnet zero", we cannot use the first subnet or the last subnet.
The first subnet ranges from 10.0.0.0 to 10.0.0.127
The last one ranges from 10.255.255.128 to 10.255.255.255
Answer "b" (10.0.0.129) doesn't fall in either of these ranges.

Looking at answer "d":
We are subnetting the 10.0.0.0 network by stealing 22 bits.
This will create 4,194,304 subnetworks of 4 IP addresses, of which the first IP is the network and the last IP is the broadcast.
So, we will have 4,194,304 subnets of 2 hosts.
Because of the "no ip subnet zero", we cannot use the first subnet or the last subnet.
The first subnet ranges from 10.0.0.0 to 10.0.0.3
The last one ranges from 10.255.255.252 to 10.255.255.255
Answer "d" (10.0.0.5) doesn't fall in either of these ranges.

Lastly, looking at answer "c":
We are subnetting the 10.0.0.0 network by stealing just 7 bits.
This will create 128 subnetworks of 131,072 IP addresses, of which the first IP is the network and the last IP is the broadcast.
So, we will have 128 subnets of 131,070 hosts.
Because of the "no ip subnet zero", we cannot use the first subnet or the last subnet.
The first subnet ranges from 10.0.0.0 to 10.1.255.255
The last one ranges from 10.254.0.0 to 10.255.255.255
Answer "c" (10.1.2.2) falls in the first subnet, and therefore will not be accepted as an IP address for an interface.

I hope that helps in some way.

That makes perfect sense man. Thanks a lot. I knew that at first when I took my ICND1, which was only a week or two ago and I passed. Very thankful for this thread and forum.