Subnetting Question

NightShade03

So this question appeared on a practice exam and I don't understand the second answer maybe someone else could clear it up for me? The question:

"Given the subnetted address space of 192.168.1.0 /29, give the E0 interface the first valid IP in the eighth subnet. Give the S0 interface the last valid IP in the twelfth subnet. The Zero Subnets are available."

Which two of the following are the correct IP and Mask configurations? Choose 2.


A. E0: 192.168.1.1 255.255.255.0
B. E0: 192.168.1.56 255.255.255.248
C. E0: 192.168.1.57 255.255.255.248
D. S0: 192.168.1.254 255.255.255.0
E. S0: 192.168.1.95 255.255.255.248
F. S0: 192.168.1.94 255.255.255.248


They say the answer is C & F.

The answer C I understand I'm just not sure where they are coming back with answer F.

Megadeth4168

The 12th subnet is 192.168.1.88 /29 so the first usable address would be 192.168.1.89 the broadcast would be 192.168.1.95 because the next subnet would begin at 192.168.1.96.

Since you cannot use teh broadcast address, the choice is 192.168.1.94 /29

NightShade03

I see how you arrived at your answer, however if the network is /29 then you would get 32 subnets, with 6 available hosts per network (but the subnets should be 8 apart, correct?) If that is the case then shouldn't the 12th subnet be:

192.168.1.88 - 192.168.1.96?

Does the zero subnet even play a role in this question?

Megadeth4168

The question says that the zero subnets are available. So, I can count 192.168.1.0 as my first usable subnet. You already know this because you agree with C as your first answer.

The 8th subnet = 192.168.1.56 with the first host being 192.168.1.57.
9th subnet = 192.168.1.64
10th subnet = 192.168.1.72
11th subnet = 192.168.1.80
12th subnet = 192.168.1.88
13th subnet = 192.168.1.96

The question asks for the last usable host in the 12th subnet. It sounds like you understand subnetting, I'm not sure what's throwing you off... Maybe it's the Subnet zero...

If the the Zero Subnets were not available, then you would basically be shifting your answers up one... The first host in 8 would be 192.168.1.65 and the last host in 12 would be 192.168.1.102.

NightShade03

I think it is the subnet zero that is throwing me off. Regardless of the subnet zero the subnetting of the network should remain the same no?

1st - 192.168.1.0 - 192.168.1.7
2nd - 192.168.1.8 - 192.168.1.15

so on and so forth....

The subnet zero just means I can use 1.0, 1.8 as a host...right? Does that also allow for the broadcast to be used or no (I would think not because its the broadcast).

NightShade03

Hold the phone it just clicked....lol

The subnets 192.168.1.0 - 1.7 would be ok because subnet zero it allowed, if it wasn't then this subnet wouldn't be allow and then the first usable subnet would be 1.8 - 1.15. Which is where you got the 1.102 as the last adress.

Does this make more sense?

Megadeth4168

Yeah, that is making sense. The Subnet Zero in your question only applies to 192.168.1.0 - 192.168.1.7 and weather or not that range is usable. Subnet Zero does not make 192.168.1.0, 192.168.1.8 ect... and available host.

NightShade03

Got it. So subnet zero only allows for creating hosts in the first subnet (if subnet zero is enabled), otherwise no hosts are allowed in the first subnet. The network id and the broadcast are never usable as hosts.

Is subnet zero something that is used alot in the real world or does it cause confusion among networks (our network is small so we don't use it).

Megadeth4168

It used to be that the IP Subnet Zero command was not on by default for Cisco Equipment. As of IOS 12.0 (I believe) IP Subnet Zero is enabled by default.

I don't think it's all that common anymore for Subnet Zero to be disabled. It should be assumed on the exam that it is enabled unless otherwise stated.

NightShade03

Ok this all makes alot more sense now....thank you for the help

Megadeth4168

No problem! That's what this board is for.