Clarification on finding number of useable subnets

AP1

I have Lammle's 5th edition book. To determine the number of useable subnets he says to use the equation 2^x (where x = number of bits turned on for the mask). So for a 255.255.255.224 (/27) (11100000) mask the equation would be 2^3 = 8 subnets. However, in the very same book (3rd edition) he writes the equation as as 2^x - 2, so 2^3 - 2 = 6 useable subnets.

Can someone explain to me which is correct, and if the latter is correct, why wo you need to subract 2 from the subnets? I already understand why for host addresses but am confused as to why the different equations.

TIA

Tricon7

AP1 wrote:

I have Lammle's 5th edition book. To determine the number of useable subnets he says to use the equation 2^x (where x = number of bits turned on for the mask). So for a 255.255.255.224 (/27) (11100000) mask the equation would be 2^3 = 8 subnets. However, in the very same book (3rd edition) he writes the equation as as 2^x - 2, so 2^3 - 2 = 6 useable subnets.

Can someone explain to me which is correct, and if the latter is correct, why wo you need to subract 2 from the subnets? I already understand why for host addresses but am confused as to why the different equations.

TIA

8 is correct; it is the total number of subnets available in your example. However, you have to subract 1 bit for the network and 1 bit for the broadcast, leaving 6 *usable* subnets. This is where the subtract-2 comes from. Make sense?

AP1

No it does not. Because I thought you only subtract 2 from the availbale numer of host addresses, to get the network address and the broadcast address. I was not under the impression that I did not need to subtract for a subnet network address and a subnet broadcast address. Do I?

Like this question from subnettingquestions.com

Question: How many subnets and hosts can you get from the network 172.29.0.0/24?

Answer: 256 subnets and 254 hosts

8 bits on for the subnet, 8 bits off for the host addresses. So 2^8 = 256 useable subnets. 2^8 - 2 = 254 useable hosts (minus 2 for the network and broadcast addresses).

Why isn't the correct answer for useable subnet hosts 254 then?

I guess what I am getting at is what is on the CCNA test? Can I use the .0 and .255 subnets? IE, would the answer on the CCNA test be 254 useable subents or 256?

EdTheLad

This topic has been discussed many times,do a search on the ccna forum for subnet-zero,you can also find a good document on this at www.cisco.com, on the exam you are told if subnet-zero is enabled or not for this type of question.

dracula

i've also got lammle 5th ed. my understanding is:

2^3 subnets if: "ip subnet-zero"

2^3 (-2) subnets if: "no ip subnet-zero"

haven't sat my ccna (yet), but from what i gather, one should assume ip subnet zero is NOT being used on the exam unless otherwise stated.

hosts are always 2 ^ n (-2) regardless of ip subnet zero (1 for network, 1 for broadcast)

i seem to recall reading somewhere that that exam objective (ip subnet zero) has changed in recent years which might account for the disparity between lammle 3rd and 5th edition.

i believe that is right but somebody pls jump in and correct if need be.

AP1

Thanks Dracula, I thought that it had something to do with ip subnet-zero, just not sure what was expected on the test.

Thanks man. I understand the hosts aspect of it, just wasn't clear on why the discrepancies between the number of useable hosts.

mwgood

This is a good read - http://www.techexams.net/forums/viewtopic.php?t=447

dracula

AP1 - read the recent "Ambiguous questions regarding subnetting (subnet zero)" thread if you haven't...

sorry - seems that, if anything, one SHOULD assume "ip subnet-zero" on the exam (unless otherwise stated). i suppose that makes sense since the command is on by default cisco IOS v12 upward.