Minus 2 rule - confusing me!!

Hi, I'm new to the forums, so hello to everyone and I'm going to post a newb question!

I'm studying for my ccna, and hope I have subnetting cracked, BUT, in the last few days, a seed of doubt has been placed in my mind!
I have always stood by the rule of 2x-2 for calculating hosts AND subnets, but whilst I've been doing loads of subnetting revision over the last few days, I found a website that generates random subnet questions and it seems to be using '2x' for calculating subnets!! Now all the books I've read have told me to knock off 1 for the top end broadcast and 1 for the low end zero network. This made me question if I was doing it right, so I asked a CCNP at work and he told me that it was definately not -2, he said -2 was only for calculating hosts, and 2x was the right rule for calculating subnets. I explained that I wanted to know for my pending ccna exam and he at no point mentioned cidr or ip subnet zero. So now I'm worried.
When I take the ccna, what formula should I use for any subnetting questions? 2x-2 or 2x? I'm hoping I'm just to use the -2 rule/formula for every question but now its got me worried!

TIA

Mors

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Comments

kplab

Traditionally, zero subnet and all-ones subnet were reserved. However, Cisco routers can be configured to override that using the command "(config)#ip subnet-zero". This command is enabled by default since IOS version 12.0. You can find more details about this topic in the following Cisco paper:

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml

In the exam, you will be told whether ip subnet-zero is enabled.

xwesleyxwillisx

Someone correct me if I'm wrong but here goes. When calculating USABLE addresses in a subnet for hosts you subtract 2 (for the network address and the broadcast address) and when calculating usable subnets you do NOT subtract 2. You used to subtract 2 for the usable subnets for the fear that routing tables would get messed up. This was before the command ip subnet zero was enabled by default in the Cisco IOS. It was thought that the zero subnet would be identical with the summary subnets associated with VLSM. VLSM is now fully supported by routing protocols such as EIGRP, OSPF, and RIP v2. It is my advice to consider subnet zero and the broadcast subnet as USABLE on your CCNA exam. Good luck!!!

mors

kplab wrote:

Traditionally, zero subnet and all-ones subnet were reserved. However, Cisco routers can be configured to override that using the command "(config)#ip subnet-zero". This command is enabled by default since IOS version 12.0. You can find more details about this topic in the following Cisco paper:

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml

In the exam, you will be told whether ip subnet-zero is enabled.

Thanks for the answer, but now my problem is, the test questions I've been using, and the sybex stuff I've been using have never mentioned about IP subnet-zero during a standard addressing question. So if they dont mention it in the question, just a usual 'how many subnets does this mask make' etc what formula should I use?

Mors

mors

xwesleyxwillisx wrote:

Someone correct me if I'm wrong but here goes. When calculating USABLE addresses in a subnet for hosts you subtract 2 (for the network address and the broadcast address) and when calculating usable subnets you do NOT subtract 2. You used to subtract 2 for the usable subnets for the fear that routing tables would get messed up. This was before the command ip subnet zero was enabled by default in the Cisco IOS. It was thought that the zero subnet would be identical with the summary subnets associated with VLSM. VLSM is now fully supported by routing protocols such as EIGRP, OSPF, and RIP v2. It is my advice to consider subnet zero and the broadcast subnet as USABLE on your CCNA exam. Good luck!!!

Arrrrgh, now I've got 2 contrasting answers on here too! Sorry guys

So do you think if they dont mention ip subnet-zero on a test question, I should just still class it as usable?

e.g
Q1. How many subnets and hosts would be created with the following network and mask x.x.x.x x.x.x.x ? Subnet zero is enabled on the router.

Q2. How many subnets and hosts would be created with the following network and mask x.x.x.x x.x.x.x ?

I this case I would use 2x for question one, 2x-2 for question two?

Still none the wiser lol

Mors

p.s
I was just looking at the prerequisite for the test and it says to be aware and able to implement the minus 2 rule, but doesn't say if that's for hosts AND subnets!! lol

CertsPlz

i think depending on how they ask the question it would be obvious what they want u to use or im alomost sure that they will tell u

EricO

They tell you in the questions whether it is disabled or not. I just finished on FEb 13th.

mors

Ok, I'll trust ya! Thanks for the input guys! Just was getting a bit edgey thats all, will teach me to use study books that are a few years old aswell!!

Mors