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Home > Archive > CCNA > July 2002 > Subnetting!
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| I feel I have a handle on subnetting. One thing that is confusing me is some Class A examples in the Todd Lammle CCNA Study Guide. With subnet mask 255.255.240.0, the book says that 10.1.0.0 is the first subnet with 10.1.16.0 being the second. How can this be? 12 bits borrowed. Why wouldn't 10.0.16.0 be the first useable subnet? If anyone has the Sybex book, it's page 153. Help! | |
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| No one wanting to tackle this one? Come on techs! | |
| rsteedma 2002-07-23, 3:01 pm |
| Given:
Class A Network 10.0.0.0
Subnet Mask 255.255.240.0
With "no ip subnet-zero"...
10.0.16.0 is the first subnet
With "ip subnet-zero"...
10.0.0.0 is the first subnet
NOTE: 10.0.0.0 as a subnet is confusing, right? That's why Cisco recommends AGAINST using "ip subnet-zero".
So you're right. Todd made a mistake.
I don't have the book in front of me, so I'm relying on what you said he said. 
Cheers,
Ron
>>>>> http://www.congonetworking.com <<<<<
Great CCNA Practice Tests - Free samples!! | |
| jeff_j_black 2002-07-23, 4:12 pm |
| I recall reading in one of Lammle's question explanations:
quote:
Remember, when working with Class A subnets, the forth octet can only have a value of 0 if either the second or third octect has a value greater than 0. Why? Lets write it out. 10.0.0.0 This is or network number. It cannot also double as the first subnet number. We must either make the second or third octet greater than 0.
Thus, our first valid subnet is 10.1.0.0
The question answer and explanation all assume 'no ip subnet zero' unless stated. | |
| rsteedma 2002-07-23, 4:51 pm |
| quote:
We must either make the second or third octet greater than 0.
Todd's on the money here - either the second or third octet must be greater than zero.
quote:
Thus, our first valid subnet is 10.1.0.0
This is where he goofed. 10.0.16.0 is less than 10.1.0.0.
Here's some router output as a check:
router(config-if)#ip add 10.0.16.1 255.255.240.0
router(config-if)#
Cisco routers accept 10.0.16.0 255.255.240.0, with or without ip subnet-zero.
Cheers,
Ron
---
Ron Steedman CCNP, CCNA, MCP, BS Computer Science
Free CCNA Practice Tests, Study Guides, & Message Boards!
http://www.congonetworking.com/ | |
| jeff_j_black 2002-07-23, 4:56 pm |
| Cool, yeah as I read the quote, I could see in his own words why 10.0.16.0 would work within the constraints of what he said. Like you, he's a smart guy. Like any of us, he can make a mistake?
Anyway on this same topic. I have been testing out with the CD that came with the Cisco Press 607 Prep book. Am I correct in thinking that a mask of 255.255.255.128 would require 'ip subnet zero'?
Otherwise, this would yield no usable subnets? | |
| rsteedma 2002-07-23, 7:14 pm |
| Todd's a good, respectable writer. His books have fewer errors than most out there and he drops by message boards from time to time to help people.
Is 255.255.255.128 allowed? That's tricky!
Class A - 8 network bits, 17 subnet bits, 7 host bits. No problem.
Class B - 16 network bits, 9 subnet bits, 7 host bits. No problem.
Class C - 24 network bits, 1 subnet bits, 7 host bits. 1 subnet bit's an issue!
Class C Networks -
CCNA Formula: (2^n-2) = 0 subnets!
* don't use all-zero or all-one subnets
This is NOT a usable subnet mask!!!!
Cisco recommended: (2^n-1) = 1 subnets!
* use all-one, but not all-zero subnets
Usable, but pointless. Why subnet at all?
ip subnet-zero Formula: (2^n) = 2 subnets!
* use all-zero and all-one subnets
Usable, and a sensible configuration.
Cheers,
Ron
---
Ron Steedman CCNP, CCNA, MCP, BS Computer Science
Free CCNA Practice Tests, Study Guides, & Message Boards!
http://www.congonetworking.com/ | |
| jeff_j_black 2002-07-23, 8:10 pm |
| Ron, thanks for clarifying on the classes. The example I did not fully detail was indeed a class C, so the ip subnet zero would be pivotal, no doubt. |
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