How Many /30 Subnets Can Be Created From One /27 Subnet?

As a developer or network engineer, you may need to occasionally await up subnet mask values and effigy out what they hateful.

To make your life easier, the freeCodeCamp community has made this elementary cheat sail. But scroll or utilise Ctrl/Cmd + f to discover the value you're looking for.

Here are the charts, followed by some explanations of what they hateful.

CIDR	Subnet mask	Wildcard mask	# of IP addresses	# of usable IP addresses
/32	255.255.255.255	0.0.0.0	1	1
/31	255.255.255.254	0.0.0.1	2	ii*
/30	255.255.255.252	0.0.0.3	4	2
/29	255.255.255.248	0.0.0.7	eight	half-dozen
/28	255.255.255.240	0.0.0.15	16	14
/27	255.255.255.224	0.0.0.31	32	thirty
/26	255.255.255.192	0.0.0.63	64	62
/25	255.255.255.128	0.0.0.127	128	126
/24	255.255.255.0	0.0.0.255	256	254
/23	255.255.254.0	0.0.1.255	512	510
/22	255.255.252.0	0.0.iii.255	ane,024	1,022
/21	255.255.248.0	0.0.7.255	2,048	2,046
/20	255.255.240.0	0.0.xv.255	iv,096	four,094
/19	255.255.224.0	0.0.31.255	eight,192	8,190
/18	255.255.192.0	0.0.63.255	sixteen,384	xvi,382
/17	255.255.128.0	0.0.127.255	32,768	32,766
/16	255.255.0.0	0.0.255.255	65,536	65,534
/fifteen	255.254.0.0	0.1.255.255	131,072	131,070
/14	255.252.0.0	0.iii.255.255	262,144	262,142
/13	255.248.0.0	0.7.255.255	524,288	524,286
/12	255.240.0.0	0.15.255.255	i,048,576	1,048,574
/xi	255.224.0.0	0.31.255.255	two,097,152	two,097,150
/10	255.192.0.0	0.63.255.255	iv,194,304	four,194,302
/9	255.128.0.0	0.127.255.255	8,388,608	viii,388,606
/viii	255.0.0.0	0.255.255.255	16,777,216	16,777,214
/7	254.0.0.0	1.255.255.255	33,554,432	33,554,430
/6	252.0.0.0	three.255.255.255	67,108,864	67,108,862
/5	248.0.0.0	7.255.255.255	134,217,728	134,217,726
/4	240.0.0.0	15.255.255.255	268,435,456	268,435,454
/3	224.0.0.0	31.255.255.255	536,870,912	536,870,910
/2	192.0.0.0	63.255.255.255	1,073,741,824	1,073,741,822
/1	128.0.0.0	127.255.255.255	two,147,483,648	2,147,483,646
/0	0.0.0.0	255.255.255.255	4,294,967,296	4,294,967,294

* /31 is a special example detailed in RFC 3021 where networks with this type of subnet mask can assign two IP addresses as a point-to-bespeak link.

And hither's a table of the decimal to binary conversions for subnet mask and wildcard octets:

	Subnet Mask		Wildcard
0	00000000	255	11111111
128	10000000	127	01111111
192	11000000	63	00111111
224	11100000	31	00011111
240	11110000	15	00001111
248	11111000	7	00000111
252	11111100	3	00000011
254	11111110	1	00000001
255	11111111	0	00000000

Note that the wildcard is just the changed of the subnet mask.

If you are new to network engineering, you lot tin get a better idea of how computer networks piece of work here.

Finally, this cheat sheet and the rest of the article is focused on IPv4 addresses, non the newer IPv6 protocol. If yous'd like to learn more than virtually IPv6, bank check out the article on computer networks above.

How Exercise IP Address Blocks Work?

IPv4 addresses like 192.168.0.ane are really just decimal representations of iv binary blocks.

Each block is 8 bits, and represents numbers from 0-255. Considering the blocks are groups of 8 bits, each block is known as an octet. And since there are iv blocks of viii bits, every IPv4 accost is 32 $.25.

For example, here's what the IP address 172.16.254.1 looks like in binary:

Source: IPv4

To convert an IP accost betwixt its decimal and binary forms, you lot can utilize this chart:

128	64	32	16	8	4	ii	1
10	10	x	x	x	x	x	ten

The chart to a higher place represents one 8 flake octive.

At present lets say you want to convert the IP accost 168.210.225.206. All you need to do is break the accost into iv blocks (168, 210, 225, and 206), and convert each into binary using the chart to a higher place.

Recall that in binary, i is the equivalent to "on" and 0 is "off". And then to catechumen the first block, 168, into binary, just start from the kickoff of the nautical chart and place a 1 or 0 in that prison cell until you get a sum of 168.

For example:

128	64	32	16	8	iv	ii	ane
1	0	one	0	i	0	0	0

128 + 32 + 8 = 168, which in binary is 10101000.

If you practise this for the residuum of the blocks, you'd get 10101000.11010010.11100001.11001110.

What is Subnetting?

If you expect at the tabular array to a higher place, information technology can seem similar the number of IP addresses is practically unlimited. After all, at that place are well-nigh iv.2 billion possible IPv4 addresses available.

But if you think about how much the internet has grown, and how many more than devices are connected these days, it might non surprise you to hear that there's already a shortage of IPv4 addresses.

Because the shortage was recognized years agone, developers came upward with a fashion to divide up an IP address into smaller networks called subnets.

This process, called subnetting, uses the host section of the IP accost to intermission information technology downward into those smaller networks or subnets.

Generally, an IP address is made upwards of network bits and host $.25:

Source: What is IPv4

So generally, subnetting does 2 things: it gives us a way to interruption up networks into subnets, and allows devices to determine whether another device/IP address is on the same local network or not.

A good mode to think almost subnetting is to moving-picture show your wireless network at habitation.

Without subnetting, every net continued device would demand its own unique IP accost.

But since yous have a wireless router, you just need one IP accost for your router. This public or external IP address is usually handled automatically, and is assigned past your internet access provider (Isp).

So every device connected to that router has its own private or internal IP address:

Source: What Is My IP Address?

Now if your device with the internal IP address 192.168.ane.101 wants to communicate with another device, it'll use the IP accost of the other device and the subnet mask.

The combination of the IP addresses and subnet mask allows the device at 192.168.i.101 to figure out if the other device is on the aforementioned network (similar the device at 192.168.i.103), or on a completely different network somewhere else online.

Interestingly, the external IP accost assigned to your router by your Internet access provider is probably function of a subnet, which might include many other IP addresses for nearby homes or businesses. And only like internal IP addresses, it too needs a subnet mask to work.

How Subnet Masks Work

Subnet masks function as a sort of filter for an IP accost. With a subnet mask, devices can look at an IP address, and effigy out which parts are the network bits and which are the host bits.

So using those things, it tin can figure out the best fashion for those devices to communicate.

If you've poked around the network settings on your router or computer, you've likely seen this number: 255.255.255.0.

If then, you lot've seen a very common subnet mask for elementary domicile networks.

Like IPv4 addresses, subnet masks are 32 bits. And just like converting an IP accost into binary, you can do the same affair with a subnet mask.

For instance, here'due south our nautical chart from earlier:

128	64	32	16	viii	four	two	ane
ten	x	ten	ten	x	10	x	x

Now let's convert the kickoff octet, 255:

128	64	32	16	8	4	ii	1
one	ane	i	ane	1	1	one	1

Pretty unproblematic, right? So any octet that's 255 is just 11111111 in binary. This ways that 255.255.255.0 is actually 11111111.11111111.11111111.00000000 in binary.

Now let's look at a subnet mask and IP accost together and calculate which parts of the IP accost are the network bits and host bits.

Here are the ii in both decimal and binary:

Blazon	Decimal	Binary
IP accost	192.168.0.101	11000000.10101000.00000000.01100101
Subnet mask	255.255.255.0	11111111.11111111.11111111.00000000

With the two laid out like this, information technology's like shooting fish in a barrel to separate 192.168.0.101 into network $.25 and host bits.

Whenever a bit in a binary subnet mask is ane, and so the same fleck in a binary IP accost is part of the network, not the host.

Since the octet 255 is 11111111 in binary, that whole octet in the IP accost is part of the network. So the first three octets, 192.168.0, is the network portion of the IP address, and 101 is the host portion.

In other words, if the device at 192.168.0.101 wants to communicate with some other device, using the subnet mask it knows that anything with the IP address 192.168.0.xxx is on the aforementioned local network.

Some other fashion to express this is with a network ID, which is just the network portion of the IP accost. And so the network ID of the address 192.168.0.101 with a subnet mask of 255.255.255.0 is 192.168.0.0.

And it's the same for the other devices on the local network (192.168.0.102, 192.168.0.103, and so on).

What Does CIDR Mean and What is CIDR Notation?

CIDR stands for Classless Inter-Domain Routing, and is used in IPv4, and more recently, IPv6 routing.

Source: Classless Inter-Domain Routing

CIDR was introduced in 1993 as a manner to wearisome the usage of IPv4 addresses, which were apace being exhausted under the older Classful IP addressing organization that the internet was first built on.

CIDR encompasses a couple of major concepts.

The first is Variable Length Submasking (VLSM), which basically allowed network engineers to create subnets within subnets. And those subnets could exist unlike sizes, so there would be fewer unused IP addresses.

The second major concept CIDR introduced is CIDR notation.

CIDR notation is really merely shorthand for the subnet mask, and represents the number of $.25 bachelor to the IP address. For example, the /24 in 192.168.0.101/24 is equivalent to the IP address 192.168.0.101 and the subnet mask 255.255.255.0.

How to Summate CIDR Noation

To figure out the CIDR notation for a given subnet mask, all you need to practise is convert the subnet mask into binary, then count the number of ones or "on" digits. For case:

Type	Decimal	Binary
Subnet mask	255.255.255.0	11111111.11111111.11111111.00000000

Because there's three octets of ones, there are 24 "on" bits significant that the CIDR notation is /24.

You can write information technology either way, but I'm certain y'all'll hold that /24 is a whole lot easier to write than 255.255.255.0.

This is usually done with an IP address, then allow'south accept a wait at the same subnet mask with an IP address:

Type	Decimal	Binary
IP address	192.168.0.101	11000000.10101000.00000000.01100101
Subnet mask	255.255.255.0	11111111.11111111.11111111.00000000

The first 3 octets of the subnet mask are all "on" bits, and then that ways that the same 3 octets in the IP address are all network bits.

Permit's take a await at the last forth octet in a fleck more detail:

Type	Decimal	Binary
IP address	101	01100101
Subnet mask	0	00000000

In this example, because all the bits for this octet in the subnet mask are "off", nosotros can be certain that all of the respective bits for this octet in the IP accost are part of the host.

When you write CIDR annotation it's usually done with the network ID. So the CIDR annotation of the IP address 192.168.0.101 with a subnet mask of 255.255.255.0 is 192.168.0.0/24.

To see more examples of how to summate the CIDR notation and network ID for a given IP address and subnet mask, bank check out this video:

Classful IP Addressing

Now that nosotros've gone over some basic examples of subnetting and CIDR, let's zoom out and wait at what'due south known as Classful IP addressing.

Back earlier subnetting was developed, all IP addresses fell into a particular class:

Source: Subnetting for dummies

Note that there are grade D and E IP addresses, just nosotros'll go into these in more detail a chip afterward.

Classful IP addresses gave network engineers a way to provide different organizations with a range of valid IP addresses.

There were a lot of issues with this arroyo that eventually atomic number 82 to subnetting. But before nosotros go into those, let's have a closer look at the unlike classes.

Course A IP Addresses

For Class A IP addresses,  the offset octet (8 bits / 1 byte) stand for the network ID, and the remaining 3 octets (24 bits / three bytes) are the host ID.

Class A IP addresses range from 1.0.0.0 to 127.255.255.255, with a default mask of 255.0.0.0 (or /8 in CIDR).

This means that Grade A addressing can accept a total of 128 (2⁷) networks and 16,777,214 (2²⁴-2) usable addresses per network.

As well, notation that the range 127.0.0.0 to 127.255.255.255 within the Grade A range is reserved for host loopback address (see RFC5735).

Class B IP Addresses

For Class B IP addresses, the start two octets (16 bits / 2 bytes) represent the network ID and the remaining ii octets (16 bits / 2 bytes) are the host ID.

Class B IP addresses range from 128.0.0.0 to 191.255.255.255, with a default subnet mask of 255.255.0.0 (or /16 in CIDR).

Form B addressing can have 16,384 (2^xiv) network addresses and 65,534 (ii¹⁶) usable addresses per network.

Class C IP Addresses

For Class C IP addresses, the kickoff three octets (24 bits / three bytes) stand for the network ID and the concluding octet (8 bits / 1 bytes) is the host ID.

Grade C IP Addresses range from 192.0.0.0 to 223.255.255.255, with a default subnet mask of 255.255.255.0 (or /24 in CIDR).

Class C translates to 2,097,152 (2²¹) networks and 254 (2^viii-2) usable addresses per network.

Course D and Form East IP Addresses

The final 2 classes are Class D and Class Due east.

Class D IP addresses are reserved for multicasts. They occupy the range from 224.0.0.0 through 239.255.255.255.

Form E IP addresses are experimental, and are anything over 240.0.0.0.

The Issue with Classful IP Addresses

The master upshot with classful IP addresses is that it wasn't efficient, and could lead to a lot of wasted IP addresses.

For example, imagine that you're part of a big organization dorsum then. Your visitor has i,000 employees, meaning that it would fall into class B.

Just if yous look above, you'll run across that a class B network tin can support up to 65,534 usable addresses. That'south fashion more than your arrangement would likely demand, even if each employee had multiple devices with a unique address.

And there was no way your organization could fall back to class C – there just wouldn't be enough usable IP addresses.

So while classful IP addresses were used around the time IPv4 addresses became widespread, it quickly became clear that a improve system would be necessary to ensure we wouldn't employ up all of the ~4.2 billion usable addresses.

Classful IP addresses haven't been used since they were replaced by CIDR in 1993, and are mostly studied to understand early internet architecture, and why subnetting is important.

I hope this cheat sheet has been a helpful reference for you

If y'all found this helpful, please share it with your friends so more people tin do good from information technology.

Also, feel free to reach out on Twitter and let me know what you think.

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Source: https://www.freecodecamp.org/news/subnet-cheat-sheet-24-subnet-mask-30-26-27-29-and-other-ip-address-cidr-network-references/

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