Subnetting made easy With an IP address of , and a subnet mask of What is the first and last assignable host of the Network (valid IP range)?. III). subnets & hosts. S it' ti f. i k i l. ▫ So, it's time for a quick review lesson in binary-to- decimal conversion. ▫ There are 8 bits in an octet & each bit can only be a 1 or. IP Subnetting Made Easy! A guide to understanding IP subnetting that won't leave you pulling you hair out. John J. Kowalski. Supernetting. .

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Page 1 of Subnetting Made Simple. IP Subnetting without Tables, Tools, or Tribulations. Larry Newcomer. The Pennsylvania State University. York Campus. –Why the mastery of IP Subnetting skills is so important in the real world. –What we know or . Done by moving the the dividing line to the right. Subnet Mask— Where Cisco Public. ▫ This is the primary tool that makes the process so easy. 10/18/Subnetting Made Easy And Other Cisco Tidbits: Subnetting Made Easy - Critically Acclaimed! 0 More Next Blog».

This information is supplied in another bit number called a subnet mask. In this example, the subnet mask is It is not obvious what this number means unless you know that in binary notation equals ; so, the subnet mask is: This gives you the following: When a packet arrives on the Almost all decimal subnet masks convert to binary numbers that are all ones on the left and all zeros on the right. Some other common subnet masks are: Decimal Binary These IP addresses are divided into classes. The most common of these are classes A, B, and C. Classes D and E exist, but are not generally used by end users. Each of the address classes has a different default subnet mask.

The most important thing to know about chopping up a network is that you can't arbitrarily pick the beginning and ending. The chopping must be along clean binary divisions.

The best way to learn this is to look at my subnet ruler and see what's a valid subnet. In Figure B, green subnets are valid and red subnets are not. Figure B The ruler was constructed like any other ruler, where we mark it down the middle and bisect it. Then, we bisect the remaining sections and with shrinking markers every time we start a new round of bisecting.

In the sample above, there were five rounds of bisections. If you look carefully at the edge of any valid green subnet blocks, you'll notice that none of the markers contained within the subnet is higher than the edge's markers.

There is a mathematical reason for this, which we'll illustrate later, but seeing it graphically will make the math easier to understand.

The role of the subnet mask The subnet mask plays a crucial role in defining the size of a subnet. Take a look at Figure C. Notice the pattern and pay special attention to the numbers in red. Whenever you're dealing with subnets, it will come in handy to remember eight special numbers that reoccur when dealing with subnet masks.

They are , , , , , , , and You'll see these numbers over and over again in IP networking, and memorizing them will make your life much easier. Figure C I've included three class sizes.

You'll see the first two classes, with host bit length from 0 to 16, most often. Private networks typically work in the 8- to bit range. Note how the binary mask has all those zeros growing from right to left. The subnet mask in binary form always has all ones to the left and all zeros to the right.

The number of zeros is identical to the subnet length. I showed only the portion of the binary subnet in the octet that's interesting, since all octets to the right consist of zeros and all octets to the left consist of ones. So if we look at the subnet mask where the subnet length is 11 bits long, the full binary subnet mask is As you can see under mask octet, the subnet mask transitions from 1 to 0 in the third octet.

The particular binary subnet mask translates directly to base form as The "mask" in subnet mask The subnet mask not only determines the size of a subnet, but it can also help you pinpoint where the end points on the subnet are if you're given any IP address within that subnet.

The reason it's called a subnet "mask" is that it literally masks out the host bits and leaves only the Network ID that begins the subnet. Once you know the beginning of the subnet and how big it is, you can determine the end of the subnet, which is the Broadcast ID.

Let's take an IP address of Note that this can be and often is written in shorthand as Inside the masking box, the 0s convert all numbers on top into zeros, no matter what the number is. When you take the resultant binary Network ID and convert it to decimal, you get One thing that's always bothered me about the way subnetting is taught is that students are not shown a simple trick to bypass the need for binary conversions when doing AND operations.

I even see IT people in the field using this slow and cumbersome technique to convert everything to binary, run the AND operation, and then convert back to decimal using the Windows Calculator.

But there's a really simple shortcut using the Windows Calculator, since the AND operator works directly on decimal numbers. I'll never understand why this isn't explained to students, because it makes mask calculations a lot easier.

Figure F Since there are 11 zeros in the subnet mask, the subnet is 11 bits long.

So the next subnet starts at If we decrease that by 1, we have To help you visualize this, Figure G shows it on my subnet ruler. Figure G IP classes made simple For an arbitrary classification of IP subnets, the creators of the Internet chose to break the Internet into multiple classes.

Note that these aren't important as far as your subnet calculations are concerned; this is just how the Internet is "laid out. Class A uses up the first half of the entire Internet, Class B uses half of the remaining half, Class C uses the remaining half again, Class D Multicasting uses up the remaining half again, and whatever is left over is reserved for Class E.

I've had students tell me that they struggled with the memorization of IP classes for weeks until they saw this simple table shown in Figure H.

This is because you don't actually need to memorize anything, you just learn the technique for constructing the ruler using half of what's available. Note that 0. All Class A addresses have their first octet between 1 to because 0 and are reserved.

Whenever you're dealing with subnets, it will come in handy to remember eight special numbers that reoccur when dealing with subnet masks. They are , , , , , , , and You'll see these numbers over and over again in IP networking, and memorizing them will make your life much easier.

You'll see the first two classes, with host bit length from 0 to 16, most often. Private networks typically work in the 8- to bit range.

Note how the binary mask has all those zeros growing from right to left. The subnet mask in binary form always has all ones to the left and all zeros to the right. The number of zeros is identical to the subnet length. I showed only the portion of the binary subnet in the octet that's interesting, since all octets to the right consist of zeros and all octets to the left consist of ones.

So if we look at the subnet mask where the subnet length is 11 bits long, the full binary subnet mask is As you can see under mask octet, the subnet mask transitions from 1 to 0 in the third octet. The particular binary subnet mask translates directly to base form as The "mask" in subnet mask The subnet mask not only determines the size of a subnet, but it can also help you pinpoint where the end points on the subnet are if you're given any IP address within that subnet.

The reason it's called a subnet "mask" is that it literally masks out the host bits and leaves only the Network ID that begins the subnet.

Once you know the beginning of the subnet and how big it is, you can determine the end of the subnet, which is the Broadcast ID.

Let's take an IP address of Note that this can be and often is written in shorthand as Inside the masking box, the 0s convert all numbers on top into zeros, no matter what the number is. When you take the resultant binary Network ID and convert it to decimal, you get One thing that's always bothered me about the way subnetting is taught is that students are not shown a simple trick to bypass the need for binary conversions when doing AND operations.

I even see IT people in the field using this slow and cumbersome technique to convert everything to binary, run the AND operation, and then convert back to decimal using the Windows Calculator.

But there's a really simple shortcut using the Windows Calculator, since the AND operator works directly on decimal numbers. I'll never understand why this isn't explained to students, because it makes mask calculations a lot easier. Figure F Since there are 11 zeros in the subnet mask, the subnet is 11 bits long. So the next subnet starts at If we decrease that by 1, we have To help you visualize this, Figure G shows it on my subnet ruler. Note that these aren't important as far as your subnet calculations are concerned; this is just how the Internet is "laid out.

Class A uses up the first half of the entire Internet, Class B uses half of the remaining half, Class C uses the remaining half again, Class D Multicasting uses up the remaining half again, and whatever is left over is reserved for Class E. I've had students tell me that they struggled with the memorization of IP classes for weeks until they saw this simple table shown in Figure H.

This is because you don't actually need to memorize anything, you just learn the technique for constructing the ruler using half of what's available. Note that 0.

All Class A addresses have their first octet between 1 to because 0 and are reserved. Class A subnets are all 24 bits long, which means the subnet mask is only 8 bits long. For example, we have the entire 3. The U. Army owns 6. Level 3 Communications owns 8. IBM owns 9. Xerox owns HP owns Apple owns All Class B addresses have their first octet between and Class B subnets are all 16 bits long, which means the subnet masks are 16 bits long.

For example, BBN Communications owns Carnegie Mellon University owns All Class C addresses have their first octet between and Class C subnets are all 8 bits long, so the subnet mask is only 24 bits long. Note that ARIN the organization that assigns Internet addresses will sell blocks of four Class C addresses only to individual companies and you have to really justify why you need 1, Public IP addresses. Also note that this isn't the old days, where blocks of The concept of subnet classes can cause harm in actual practice.

I've actually seen people forget to turn classes off in their old Cisco router and watch large subnet routes get hijacked on a large WAN configured for dynamic routing whenever some routes were added.

All newer Cisco IOS software versions turn off the concept of subnet classes and uses classless routing by default. This is done with the default command "IP Classless.