Using show ip route and Subnet Math to Find the Best Route.

[Arcaneuk10]

Dear all

Please assist in this.

I am answering a test question. which is the answer below

100.100.0.0/14 (range 100.100.0.0 - 100.103.255.255)

so I began by looking for the subnet id which I worked out as:

/14 = 255.252.0.0

I then looked for the interesting octet which is 252, the magic number is therefore 256-252 = 4

so now in multiples of 4, I need to find the number that does not go over .100 which I make to be 96, so the subnet id is 100.96.0.0 

I then look for the subnet broadcast, using the subnet id, 100.96.0.0, again the interesting octet is 252 and the magic number is 4, therefore 96+4-1 =99, therefore the subnet broadcast is 100.99.255.255, and the range I worked out as 100.96.0.1 - 100.99.255.254 however as you can see from above the test answer is:

100.100.0.0/14 (range 100.100.0.0 - 100.103.255.255), any pointers on what I am doing wrong?

Much regards

Arcaneuk10

[paul driver]

Hello

Your subnetwork is incorrect it should be 100.100.0.0  as 4/100 =25 and 25*4 =100

So 4 goes into 100 exactly so you use 100 not 96  you only use the lower value if the bit value didn’t match

1) subnet mask

Network 100.100.0.0/14
                8      6

128 1
192 2
228 3
240 4
248 5
252 6 <---
254 7
255 8

100.100.0.0
255.252.0.0

2) ip range

4/100 = 25
4*25 = 100

Subnet 100.100.0.0
First ip 100.100.0.1
Broadcast 100.103.255.255   <- (256-252 =4) 100 + (4-1) =3
Last ip 100.103.255.254

Ip range = 100.100.0.1 - 100.103.255.254
Subnet = 100.100.0.0
Broadcast= 100.103.255.255

[Arcaneuk10]

Hello Paul

Many thanks for your help in this, it makes sense now, I thought that you did not go over the interesting octet, which was 100, but as long as it divides exactly into the interesting octet, it is fine. that is a valuable lesson I learnt, which means that I need to read up about subnetting again before I take the exam, the explanation you gave was very concise and made sense to me.

Much regards

Arcaneuk10