4 Replies Latest reply: Mar 27, 2018 5:42 PM by ciscodaze1

# Subnetting question

Hi guys,

I have a subnetting question that hope you can help me with. The question is the following:

I am given IP address 192.168.15.30 and subnet mask 255.255.255.252 and I need to find out what is the subnet id for this IP.

Before anything else, I'd like to explain that the method I am using for practising subnetting (and which I am pretty comfortable with) focuses on the use of 8, 16 and 32 values to find out what the increments are for the subnets.

In this example, 255.255.255.252 is /30 in CIDR notation. This means that my increment would be 2^ (32-30) = 2^2= 4.

Based on that increment, I would list the network ranges (adding the increment on the last octet) as follows:

192.168.15.0

192.168.15.4

192.168.15.8

..................

192.168.15.224

192.168.15.228 (192.168.15.231)

192.168.15.232

Based on my results, the subnet id for 192.168.15.30 would be 192.168.15.228.

I see this method very easy to understand and to implement to get fast results in most of the examples. However, in a case like this example (where you have to increment all the way from 0 to 228) is not fast enough for me). I tried to say "ok, 4 times 50 is 200, 4 times 60 is 240 so my subnet is somewhere around those limits". From there, I keep looking for the right multiple of 4 to get to my value between 50 and 60.

This makes me spend a lot of time in trying to find this number.

In order to get accurate at subnetting with this method, I have learned by heart the mutliples for 128, 64,32 and 16 up to 256 (well, really easy for 128 as there's only 0 and 128) but for increments with values 8,4 and 2 I struggle when the increment for the subnet is far away from 0.

Do you guys have any method or technique that might help for these types of scenarios?

Regards,

Daniel

• ###### 2. Re: Subnetting question

you are right, 225.225.225.252 is a CIDR OF /30 and an increment of 4.

however, the interesting octet is the fourth octet. this means u increment in the 4th octet of the given IP address viz:

subnets numbers   0    4    8    12    16    20    24    28    32.....

first IP                      1    5    9    13    17    21   25    29     33

Last IP                     2    6    10    14   18   22    26   30     34

Broadcast Ip           3    7   11    15   19    23    27    31    35...

As you can see your IP add 192.168.15.30 sits as the last ip in the 192.168.15.28/30 subnet.

• ###### 3. Re: Subnetting question

Just wanted to point out that this question is more than 3 years old, and the guy hasn't logged on since 2016.

• ###### 4. Re: Subnetting question

Once the first person performs the resurrection, it justs goes on and on...