the term octet is used to describe what size of binary number?

Networking Basics: What is IPv4 Subnetting?

How to Subnet IPv4

The quick definition: Subnetting is the process of taking a network and splitting information technology into smaller networks, known as subnets. It's used to free upward more public IPv4 addresses and segment networks for security and easier management.

A quick rundown of IP addresses and binary

To empathize subnetting, you should first understand the decimal and binary structure of an IP address.

Let's start with the basics. Hither'southward what an IP address looks like: 192.168.i.20

An IPv4 address is a 32-scrap number. To make addresses more than straightforward, they are divided into four viii-bit numbers — or octets — separated past a decimal point. These octets range in number from zero to 255.

Why do octets merely go upward to 255? Because they're binary.

The biggest IP address possible is 255.255.255.255

In binary, this IP address looks like this: 11111111.11111111.11111111.11111111

Note that there are eight numbers betwixt the decimal points. Each number represents a bit. Hence the term octet or the 8-flake number group.

Binary corresponds to this table:

128

64

32

sixteen

eight

4

2

1

Permit'southward employ this binary number, for example: 10000001

Every 1 in a binary number "turns on" the number in its position. So, one in the starting time and terminal positions "turn on" 128 and i.

128	64	32	16	8	four	2	1
1	0	0	0	0	0	0	1

Add up all the positions to get the decimal number: 128 + ane = 129

When all the positions are "turned on," they add together up to 255.

You can come across how it works here. These are the most common octets you'll encounter in subnetting:

	128	64	32	xvi	8	4	ii	ane
255	one	1	ane	1	1	1	1	1
254	ane	1	1	one	1	1	1	0
252	i	1	1	1	one	1	0	0
248	1	1	ane	ane	1	0	0	0
240	1	1	1	1	0	0	0	0
224	1	1	i	0	0	0	0	0
192	ane	1	0	0	0	0	0	0
128	ane	0	0	0	0	0	0	0

How to ascertain the network portion of a subnet IP address

During the early stages of the cyberspace, organizations assigned IP addresses like crazy until nosotros well-nigh ran out. Luckily, the designers of IP addressing came up with a way to terminate this wasteful exercise: Dividing networks using subnetting.

The process of taking an all-encompassing network and splitting into smaller networks is known equally subnetting — and it'due south freeing upwardly more public IPv4 addresses.

There are two parts to an IP address: The network portion and the host portion.

Information technology'due south like the address for a house. The network portion is like the city, land, and nil code. The host portion is like the house and street number.

A subnet defines the number of bits, out of 32, used for the "network portion" of the address. Subnet masks can besides be defined in a more mutual 'slash' representation, known as CIDR notation. In the following table, the cherry-red digits represent the $.25 used for the network. The black digits will be used for device IP addresses. Annotation that the 255.0.0.0 mask tin also exist represented as a '/8' because it reserves 8 bits of the overall 32 bits used to describe an IPv4 address every bit the network portion.

Bits used for mask	Default netmask	Subnet binary
/viii	255.0.0.0	11111111.00000000.00000000.00000000
/xvi	255.255.0.0	11111111.11111111.00000000.00000000
/24	255.255.255.0	11111111.11111111.11111111.00000000

For example, you lot might accept a network with devices (known as hosts) with the post-obit IP addresses:

Network: 172.16.56.0
Reckoner ane: 172.sixteen.56.twoscore
Computer 2: 172.16.56.55
Printer i: 172.16.56.100

In this case, we're using 24 bits (or three octets) for the network. Notice that every host device in the network has the same beginning iii octets. That's the network portion of the IP address with a /24 mask.

IP accost: 172.16.56.40
Mask: 255.255.255.0
Binary mask: 11111111.11111111.11111111.00000000

The last octet is the host portion of the IP accost. That's where you'd assign your devices. In this case, you could assign up to 254 hosts. (More on that later.)

IP address: 172.16.56.forty
Mask: 255.255.255.0
Binary mask: 11111111.11111111.11111111.00000000

Let'south look at the tabular array again. If it were /16, so the first two octets would exist the network portion, and the host portion would occupy the terminal ii octets.

/16

255.255.0.0

11111111.11111111.00000000.00000000

If information technology were an /8 network, then only the first octet would be the network portion.

/8

255.0.0.0

11111111.00000000.00000000.00000000

These are the near common masks because they're the simplest, simply when you lot need more than ane network, you have to subnet. Subnetting enables you to cull the number of $.25 to use for the Network portion. You can even steal $.25 from the host portion for the network.

Here'south what the full subnet mask table looks like. In this table, 1s represent the network portion, and 0s represent the host portion.

Slash	Netmask	1st Octet	2nd Octet	3rd Octet	4th Octet
/30	255.255.255.252	11111111	11111111	11111111	11111100
/29	255.255.255.248	11111111	11111111	11111111	11111000
/28	255.255.255.240	11111111	11111111	11111111	11110000
/27	255.255.255.224	11111111	11111111	11111111	11100000
/26	255.255.255.192	11111111	11111111	11111111	11000000
/25	255.255.255.128	11111111	11111111	11111111	10000000
/24	255.255.255.0	11111111	11111111	11111111	00000000
/23	255.255.254.0	11111111	11111111	11111110	00000000
/22	255.255.252.0	11111111	11111111	11111100	00000000
/21	255.255.248.0	11111111	11111111	11111000	00000000
/twenty	255.255.240.0	11111111	11111111	11110000	00000000
/xix	255.255.224.0	11111111	11111111	11100000	00000000
/18	255.255.192.0	11111111	11111111	11000000	00000000
/17	255.255.128.0	11111111	11111111	10000000	00000000
/16	255.255.0.0	11111111	11111111	00000000	00000000
/15	255.254.0.0	11111111	11111110	00000000	00000000
/14	255.252.0.0	11111111	11111100	00000000	00000000
/13	255.248.0.0	11111111	11111000	00000000	00000000
/12	255.240.0.0	11111111	11110000	00000000	00000000
/11	255.224.0.0	11111111	11100000	00000000	00000000
/x	255.192.0.0	11111111	11000000	00000000	00000000
/9	255.128.0.0	11111111	10000000	00000000	00000000
/eight	255.0.0.0	11111111	00000000	00000000	00000000
/7	254.0.0.0	11111110	00000000	00000000	00000000
/6	252.0.0.0	11111100	00000000	00000000	00000000
/5	248.0.0.0	11111000	00000000	00000000	00000000
/4	240.0.0.0	11110000	00000000	00000000	00000000
/three	224.0.0.0	11100000	00000000	00000000	00000000
/2	192.0.0.0	11000000	00000000	00000000	00000000
/one	128.0.0.0	10000000	00000000	00000000	00000000

What are IP address classes?

To complicate things further, IP addresses have five classes, only just three are applicable to subnetting — A, B, C.

Here are the IP address ranges by course:

Form A = i.0.0.0 to 127.0.0.0

Class B = 128.0.0.0 to 191.255.0.0

Class C = 192.0.0.0 to 223.255.255.0

Remember these IP addresses are represented in binary.

Hither are the largest subnet IP addresses in these ranges:

Form A	127.0.0.0	01111110	00000000	00000000	00000000
Class B	191.255.0.0	10111111	11111111	00000000	00000000
Course C	223.255.255.0	11011111	11111111	11111111	00000000

This is important to know because information technology affects the number of hosts and subnets bachelor in a network.

Detect that Course A addresses provide the nearly room for host addresses (the black digits). That's because the network portion only occupies the kickoff octet. Nigh large enterprises apply Form A IP addresses for this reason. You tin can connect more than devices to a Class A network than a Class C.

Class A

127.0.0.0

01111111

00000000

00000000

00000000

In every class, you can steal $.25 from the hosts to create more than subnets, but you're too reducing the number of hosts. Notice how stealing only one bit for the network drops the number of hosts significantly.

Grade A Subnet Netmasks and Hosts

Network Bits	Subnet Mask	Number of Subnets	Number of Hosts
/viii	255.0.0.0	0	sixteen,777,214
/nine	255.128.0.0	0	eight,388,606
/ten	255.192.0.0	2	4,194,302
/xi	255.224.0.0	half dozen	2,097,150
/12	255.240.0.0	14	1,048,574
/xiii	255.248.0.0	30	524,286
/14	255.252.0.0	62	262,142
/xv	255.254.0.0	126	131,070
/16	255.255.0.0	254	65,534
/17	255.255.128.0	510	32,766
/18	255.255.192.0	1,022	16,382
/19	255.255.224.0	2,046	8,190
/20	255.255.240.0	four,094	iv,094
/21	255.255.248.0	8,190	two,046
/22	255.255.252.0	16,382	1,022
/23	255.255.254.0	32,766	510
/24	255.255.255.0	65,534	254
/25	255.255.255.128	131,070	126
/26	255.255.255.192	262,142	62
/27	255.255.255.224	524,286	30
/28	255.255.255.240	1,048,574	14
/29	255.255.255.248	two,097,150	half-dozen
/thirty	255.255.255.252	4,194,302	ii

Form B IP addresses offering fewer hosts than Grade A considering its network portion occupies the beginning two octets.

Class B

191.255.0.0

10111111

11111111

00000000

00000000

Class B Subnet Netmasks and Hosts

Network Bits	Subnet Mask	Number of Subnets	Number of Hosts
/sixteen	255.255.0.0	0	65,534
/17	255.255.128.0	0	32,766
/18	255.255.192.0	2	sixteen,382
/xix	255.255.224.0	6	eight,190
/20	255.255.240.0	14	four,094
/21	255.255.248.0	30	2,046
/22	255.255.252.0	62	1,022
/23	255.255.254.0	126	510
/24	255.255.255.0	254	254
/25	255.255.255.128	510	126
/26	255.255.255.192	ane,022	62
/27	255.255.255.224	two,046	30
/28	255.255.255.240	four,094	14
/29	255.255.255.248	8,190	6
/30	255.255.255.252	16,382	2

Course C IP addresses offering the fewest hosts because the network portion occupies 3 octets.

Course C

223.255.255.0

11011111

11111111

11111111

00000000

You might find that the default IP accost your dwelling router uses falls into the Class C category. This is a special subnet reserved for individual IP addresses, you tin read why in the Network Address Translation article.

Grade C Subnet Netmasks and Hosts

Network Bits	Subnet Mask	Number of Subnets	Number of Hosts
/24	255.255.255.0	0	254
/25	255.255.255.128	0	126
/26	255.255.255.192	2	62
/27	255.255.255.224	6	30
/28	255.255.255.240	14	14
/29	255.255.255.248	30	6
/30	255.255.255.252	62	2

These standards make subnetting a picayune easier. For instance, if you cull a Form 'C' address, you know that it uses at least 24 bits (/24) of the 32 available bits for the network portion of the address.

How to subnet in IPv4

At present that we know most classes, binary, and subnets. Let'south swoop into a subnet.

Here'due south the IP accost nosotros'll use: 43.17.255.71/27

Here's what it looks similar in binary:

00101011.000100011.111111.01000111

From the IP address nosotros already know two things:

• It's a Class A IP Address
• It must have at to the lowest degree 8 network bits, but nosotros're giving it 27 $.25

In that case, we know the network portion of the subnet will occupy these bits:

/27

255.255.255.224

11111111

11111111

11111111

11100000

Let's reverse engineer this final octet to make up one's mind the network portion of the address or what the subnet is for this address.

Here'due south what we desire to do:

1. Determine the number of allowed subnets using the /27 network mask
2. Make up one's mind what subnet the accost lies in

Here'southward an example:

ane. Determine number of immune subnets using /27 network mask.

Here's the binary representation of the possibilities for the terminal octet with a /27 mask:

000|0 0000	001|0 0000	010|0 0000	011|0 0000	100|0 0000	101|0 0000	110|0 0000	111|0 0000
0	32	64	96	128	160	192	224

This gives us eight possible subnets with the /27 mask.

2. How to decide what subnet your IP address lives

At present, let'southward observe the subnet accost where this IP address resides.

Remember that the IP address is 43.17.255.71

We are simply looking at the final octet because the first iii octets are the network portion.

43.17.255.71

We just take to take a await at our table again. 71 falls in a higher place the 43.17.255.64 subnet and below the 43.17.255.96 subnet. And so it belongs in the 43.17.255.64 subnet.

Subnet	Last Octet		Cake size	IP Accost
1	0	+	32	43.17.255.0
two	32	+	32	43.17.255.32
3	64	+	32	43.17.255.64
4	96	+	32	43.17.255.96
v	128	+	32	43.17.255.128
6	160	+	32	43.17.255.160
7	192	+	32	43.17.255.192
8	224	+	32	43.17.255.224

Nosotros now accept the subnet for 43.17.255.71: 43.17.255.64.

That seemed like a lot of work, right? If you are logged in to a Cisco router that's connected to the subnet, there's a more efficient and easier way to get the subnet. Using the command "show ip road" in the Cisco router control line, the output includes the subnet for the IP accost.

Of course, y'all won't always have the control line at your fingertips. That'southward why it'southward of import to understand how IPv4 addressing works and be able to calculate subnets. Using a chart similar the i higher up and the method we covered makes that process easier. Yous'll exist able to identify addresses from subnets in minutes, fifty-fifty seconds. Who knows, at that betoken, yous'll want to tackle IPv6 subnetting.

For now, Keith Barker explains how to reverse engineer an IPv4 address.

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