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Another Powerful Method for Subnetting

By: JBingham

June 17, 2016

Another Powerful Method for Subnetting - Cybrary

Here's something that I learned from my CISCO class that makes things a lot easier.You're given an IP range of 192.168.0.0 and must create a domain that has five subnets - pockets of devices on the network that are separate from each other so that no one can interfere with another departments data or devices. You know the first part, five domains, now you'll need to know how many host are going to be in each domain. They are:

Host A (Faculty) – 50 hosts (plus 1 for fast Ethernet Interface)

Host B (Students) – 100 hosts (plus 1 for fast Ethernet Interface)

Host C (Staff) – 26 hosts (plus 1 for fast Ethernet Interface)

WAN1 – Router A serial connection to Router C serial (2 hosts)

WAN2 – Router B serial connection to Router C serial (2 hosts)

That's the easy part. Now, let's set up a chart, which you should memorize for the exam:

Bits	128	64	32	16	8	4	2	1
CIDR	/25	/26	/27	/28	/29	/30	/31	/32
Subnet Mask	128	192	224	240	248	252	254	255
Hosts	126	62	30	14	6	2

If you can memorize the above chart, then you should be able to ace the section on subnetting. When you go into the exam, sit down and do a brain dump. This is one of the first things that you'll want to write down prior to logging onto the computer to begin your exam.Now that you have that written down, you'll come to the question at hand. How do you go about breaking up the IP address given you into five different domains? You'll need to create another chart to assist you with this; there are some clues when you're doing this.Here's the next chart:

Useable Host	Network	1^st Address	Last Address	Broadcast Address	Subnet Mask
100
50
26
2
2

Have you noticed something? I've inputted, from highest to lowest, the number of hosts that we'll need to create our network with the requested subnets. From here on, I'll be going one line at a time, so it'll make it easy to follow.The first thing that you'll input is the IP address that you were given, 192.168.0.0:

Useable Host	Network	1^st Address	Last Address	Broadcast Address	Subnet Mask
100	192.168.0.0

Now, put in the 1^st address that you will use:

Useable Host	Network	1^st Address	Last Address	Broadcast Address	Subnet Mask
100	192.168.0.0	192.168.0.1

Notice, you have 100 hosts to fulfill, but, if you look at the chart that was first created, there is no 100 address. You cannot go down, but you can go up, so you go to the column of 126 hosts and put that to the 1^st address.

Useable Host	Network	1^st Address	Last Address	Broadcast Address	Subnet Mask
100	192.168.0.0	192.168.0.1	192.168.0.126

Of course, your broadcast address only goes up one bit, so input that:

Useable Host	Network	1^st Address	Last Address	Broadcast Address	Subnet Mask
100	192.168.0.0	192.168.0.1	192.168.0.126	192.168.0.127

Here's the fun part: you have a need of 100 hosts, but, you chose 126, what do you think would be the subnet mask? Look at the chart, it is right there:

Useable Host	Network	1^st Address	Last Address	Broadcast Address	Subnet Mask
100	192.168.0.0	192.168.0.1	192.168.0.126	192.168.0.127	255.255.255.128

Looks simple, doesn’t it? Now, don’t get too confident. Let's fill in the rest of the address and see if you notice something that will help you:

Useable Host	Network	1^st Address	Last Address	Broadcast Address	Subnet Mask
100	192.168.0.0	192.168.0.1	192.168.0.126	192.168.0.127	255.255.255.128
50	192.168.0.128	192.168.0.129	192.168.0.190	192.168.0.191	255.255.255.192
26	192.168.0.192	192.168.0.193	192.168.0.222	192.168.0.223	255.255.255.224
2	192.168.0.224	192.168.0.225	192.168.0.226	192.168.0.227	255.255.255.252
2	192.168.0.228	192.168.0.229	192.168.0.230	192.168.0.231	255.255.255.252

You've now filled in the complete graph of the breakdown of your IP address. Did you notice a couple of things? This will help you to check yourself if you're doing it properly. Look close. The Network and last address is always an even number, the first address and broadcast address is always odd. Also, the IP address that follows the broadcast address is just the next IP address.These little bits of clues will help you in creating subnets for your domain. Once I saw the pattern, I was never wrong again in class.I hope that I was able to explain it so you could follow along. I've passed on the tools that I've found helpful and hope that you'll also find them just as useful.