Confused in getting the number of networks

chinaman

I am confused regarding how to get the number of subnets and host.
In the www.subnettingquestion.com site:
How many subnet and host in 192.168.88.0/29?
So my answer is 30 subnet and 6 hosts but it gives me 32 subnet and 6 hosts.
Is my formula correct 2 to n – 2 = network or subnet
2 to n – 2 = number of hosts

eurotrash

you don't subtract the 2 for the network portion.

chinaman

But in CCNA INTRO chapter 12 it subtr. 2 on the network or subnet portion also in Sybex 5th editon.

I think you need to subtract 2 on the network protion.

eurotrash

RFC1878 wrote:

For the sake of completeness within this memo, tables 2-1 and 2-2 illistrate some options for subnet/host partions within selected block sizes using calculations which exclude all-zeros and all-ones subnets [2]. Many vendors only support subnetting based upon this premise. This practice is obsolete! Modern software will be able to utilize all definable networks.

http://www.ietf.org/rfc/rfc1878.txt

i guess it depends on the vendor. Microsoft includes the all 1's and 0's.

chinaman

So base on the link that you gave me, I need to subtruct -2 on the network portion.


if I have class C borrowed 2 bits the number of network or subnet is 2 and left 6 bits for host which is 62 hosts.

Thanks

dolphin_m70

chinaman wrote:

I am confused regarding how to get the number of subnets and host.
In the www.subnettingquestion.com site:
How many subnet and host in 192.168.88.0/29?
So my answer is 30 subnet and 6 hosts but it gives me 32 subnet and 6 hosts.
Is my formula correct 2 to n – 2 = network or subnet
2 to n – 2 = number of hosts

network = 24
host = 32 -29 = 3
subnet = 32 - (network + host) = 24 + 3 =32 - 27 = 5

no of subnet = 2^no of subnet bits - 2 = 2^5 - 2 = 30
no of host = 2^no of host bits - 2 = 2 ^3 -2 = 6

eurotrash

take a look here http://www.experts-exchange.com/Networking/Q_21731445.html

edit: go here and enter your network address, class (C), and desired number of subnets (32) and it will give you a /29 mask.

dolphin_m70

u r rgt omni , but i thk they have added the 0 and 1 subnet as well, normally it is "discouraged" as far as i know tht is the (-2)

eurotrash

it's true that cisco discourages it, but they also say

cisco wrote:

It should be noted that even though it was discouraged, the entire address space including subnet zero and the all-ones subnet have always been usable. The use of the all-ones subnet was explicitly allowed and the use of subnet zero is explicitly allowed since Cisco IOS Software Release 12.0. Even prior to Cisco IOS Software Release 12.0, subnet zero could be used by entering the ip subnet-zero global configuration command.

http://www.cisco.com/warp/public/105/40.html

duncnz

So what line do we take for exam time?

x_Danny_x

what? so you dont subract 2 for the network??? come on, please say it aint so!!!

So what does Cisco say when it comes to the exam questions about finding out how many networks there are?

chinaman

Maybe on that site they include the subnet zero which cisco discourage and on my book it is mentioned that you cannot use the subnet zero though it is usable. But better on the exam not to choose it.

So 2 to n - 2 is the correct formula to get the number of network ( 0 subnet is not included

GRRR........CONFUSE

So subtruct or not to subtruct 2 ??????

HEY GURU's need your help....

eurotrash

just be sure to use whatever method cisco requires, i'm not doing cisco stuff atm but there's the cisco way which you will have to use for the cisco exams, and the microsoft way for the microsoft exams (which includes the all 1's + 0's as subnets).

then it depends if they are asking "how many subnets" or "how many usable subnets", etc.

EricO

I just finished ICND last Month. If cisco wants you to consider the Zero subnet then they will say "Ip subnet zero has been configured." Don't over think this.

dolphin_m70

I think that in the exam , they specifally mention about the use of "IP subnet zero"

else it is "discouraged" as such ,that is what the ICND book of Cisco press by Wendell Odom says