A question about custom subnetting

Tricon7 · January 2007

I'm getting the hang of custom subnetting, but one aspect is still confusing to me. To give an example, if you only need 1 usable subnet and the network address is 195.223.50.0, then you'll borrow 2 bits, the total number of subnets will be 4, the number of usable subnets will be 2, etc. However, when the problem asks what is the 2nd usable subnet range, and what are the assignable addresses for the 2nd usable subnet, I get fuzzy with it.

I could use some help. Thanks.

benbuiltpc · January 2007

195.223.50.0 /24 to start

195.223.50.0 /26 = borrowed 2 bits = 255.255.255.192 netmask

256-192 = block size of 64

First host range: 0-63 (AKA subnet 0, not used in this example)
Second host range: 64-127 (Valid hosts are .65-.126)
Third host range 128-191 (Valid hosts re .129-190)
Fourth host range: 192-254 (Last subnet, not used)

So, you have here 2 usable subnets with 62 usable hosts per subnet.

There's a binary method too... personally I just find the block size and get all the ranges.

Kaminsky · January 2007

Tricon7 wrote:

However, when the problem asks what is the 2nd usable subnet range, and what are the assignable addresses for the 2nd usable subnet, I get fuzzy with it.

I think it's the use of subnet zero which is confusing not only you but many others. Apparantly on the exam, the question will state whether the use of subnet zero is supported.

It would be good if someone who has recently sat this exam and come across this issue could post the give clarity to this.

aquilla · January 2007

Kaminsky wrote:

Tricon7 wrote:

However, when the problem asks what is the 2nd usable subnet range, and what are the assignable addresses for the 2nd usable subnet, I get fuzzy with it.

I think it's the use of subnet zero which is confusing not only you but many others. Apparantly on the exam, the question will state whether the use of subnet zero is supported.

It would be good if someone who has recently sat this exam and come across this issue could post the give clarity to this.

I was always told that ip subnet zero is off unless Cisco specifically state otherwise during the exam. I've been doing a couple of practice exams on the Cisco website - http://www.cisco.com/go/prepcenter and it's either not mentioned (in which case assume you can't use it), or it's mentioned and you adjust accordingly.

Unless it's specifically mentioned that ip subnet zero is on, discard the first subnet and discard the last subnet.

With ip subnet zero turned OFF, a /26 (255.255.255.192) would look like:

First valid subnet
195.223.50.64 - 195.223.50.127
Valid Hosts - 195.223.50.65 - 195.223.50.126

Second valid subnet
195.223.50.128 - 195.223.50.191
Valid Hosts - 195.223.50.129 - 195.223.50.190

With ip subnet zero turned ON, a /26 (255.255.255.192) would look like:

First valid subnet
195.223.50.0 - 195.223.50.63
Valid Hosts - 195.223.50.1 - 195.223.50.62

Second valid subnet
195.223.50.64 - 195.223.50.127
Valid Hosts - 195.223.50.65 - 195.223.50.126

Third valid subnet
195.223.50.128 - 195.223.50.191
Valid Hosts - 195.223.50.129 - 195.223.50.190

Fourth valid subnet
195.223.50.192 - 195.223.50.255
Valid Hosts - 195.223.50.193 - 195.223.50.254

A question about custom subnetting

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