1
Ths is an Internet Protocol (IPv4) Subnet Chart. You can use this to quickly look up how your
might need to subnet your network. At the bottom there is a quick how

to on calculating
subnets.
For more information on subnetting, see
RFC 1817
and
RFC 1812
.
Class address ranges:
Class A = 1.0.0.0 to 126.0.0.0
Class B = 128.0.0.0 to 191.255.0.0
Class C = 192.0
.1.0 to 223.255.255.0
Reserved address ranges for private (non

routed) use (see
RFC 1918
):
10.0.0.0

> 10.255.255.255
172.16.0.0

> 172.31.255.255
192.168.0.0

> 192.168.255.25
5
Other reserved addresses:
127.0.0.0 is reserved for loopback and IPC on the local host
224.0.0.0

> 239.255.255.255 is reserved for multicast addresses
Chart notes:
Number of Subnets

"( )" Refers to the number of effective subnets, since the use o
f
subnet numbers of all 0s or all 1s is highly frowned upon and RFC non

compliant.
Number of Hosts

Refers to the number of effective hosts, excluding the network and
broadcast address.
Class A
Network Bits
Subnet Mask
Number of Subnets
Number of Hosts
/8
255.0.0.0
0
16777214
/9
255.128.0.0
2 (0)
8388606
/10
255.192.0.0
4 (2)
4194302
/11
255.224.0.0
8 (6)
2097150
/12
255.240.0.0
16 (14)
1048574
/13
255.248.0.0
32 (30)
524286
/14
255.252.0.0
64 (62)
262142
/15
255.254
.0.0
128 (126)
131070
/16
255.255.0.0
256 (254)
65534
2
/17
255.255.128.0
512 (510)
32766
/18
255.255.192.0
1024 (1022)
16382
/19
255.255.224.0
2048 (2046)
8190
/20
255.255.240.0
4096 (4094)
4094
/21
255.255.248.0
8192 (8190)
2046
/22
255.255.252.0
16384 (16382)
1022
/23
255.255.254.0
32768 (32766)
510
/24
255.255.255.0
65536 (65534)
254
/25
255.255.255.128
131072 (131070)
126
/26
255.255.255.192
262144 (262142)
62
/27
255.255.255.224
524288 (524286
)
30
/28
255.255.255.240
1048576 (1048574)
14
/29
255.255.255.248
2097152 (2097150)
6
/30
255.255.255.252
4194304 (4194302)
2
Class B
Network Bits
Subnet Mask
Number of Subnets
Number of Hosts
/16
255.255.0.0
0
65534
/17
255.255.12
8.0
2 (0)
32766
/18
255.255.192.0
4 (2)
16382
/19
255.255.224.0
8 (6)
8190
/20
255.255.240.0
16 (14)
4094
/21
255.255.248.0
32 (30)
2046
/22
255.255.252.0
64 (62)
1022
/23
255.255.254.0
128 (126)
510
/24
255.255.255.0
25
6 (254)
254
/25
255.255.255.128
512 (510)
126
/26
255.255.255.192
1024 (1022)
62
/27
255.255.255.224
2048 (2046)
30
/28
255.255.255.240
4096 (4094)
14
3
/29
255.255.255.248
8192 (8190)
6
/30
255.255.255.252
16384 (16382)
2
Cl
ass C
Network Bits
Subnet Mask
Number of Subnets
Number of Hosts
/24
255.255.255.0
0
254
/25
255.255.255.128
2 (0)
126
/26
255.255.255.192
4 (2)
62
/27
255.255.255.224
8 (6)
30
/28
255.255.255.240
16 (14)
14
/29
255.255.255.248
32 (30)
6
/30
255.255.255.252
64 (62)
2
Supernetting (CIDR) Chart
CIDR

Classless Inter

Domain Routing.
Note: The Number of Class C networks must be contiguous.
For example, 192.169.1.0/22 represents the following block of addresses:
192.169.
1.0, 192.169.2.0, 192.169.3.0 and 192.169.4.0.
Class C
CIDR Block
Supernet Mask
Number of Class C Addresses
Number of Hosts
/14
255.252.0.0
1024
262144
/15
255.254.0.0
512
131072
/16
255.255.0.0
256
65536
/17
255.255.128.0
128
32768
/
18
255.255.192.0
64
16384
/19
255.255.224.0
32
8192
/20
255.255.240.0
16
4096
/21
255.255.248.0
8
2048
/22
255.255.252.0
4
1024
/23
255.255.254.0
2
512
4
Quick Subnetting How

To (Thanks to
Jason
@
GeekVenue.)
[Understanding decimal

Base 10]
The first thing you must know is that the common number system used world wide is the
decimal system
(otherwise known a
s
base 10
). What makes the decimal system a base 10
system is that it is
based on grouping numbers by 10's
. It is believed that the system
evolved because we have ten fingers and ten toes which over the years we have used for
counting. I use mine all the t
ime (grin). We name the ten digits: zero, one, two, three, four,
five, six, seven, eight and nine.
The decimal system has a
1
's place, a
10
's place, a
100
's place, a
1000
's place and so on.
We say the number places are grouped by 10's because
multiplying
each number place by
10 gives you the next number place
. So: 1x10=10 (the 10's place), 10x10=100 (the 100's
place), 100x10=1000 (the 1000's place) etc.
Let's look at the decimal number
103
by place.
103
<

read from right to left
We have a
3
in the
1's
place
We have a
0
in the
10's place
We have a
1
in the
100's place
Thus:
100+0+3=103
By now you probably feel like you have attended Kindergarten for the second time in your life?
Sorry about that but it is very important that you understand the concept o
f what a number
system is, and what it is based on before we look at binary.
[Understanding binary

base 2]
Binary is a
base 2
system, and thus groups numbers by 2's and not by 10's like the decimal
system. We name the two digits: zero and one. The b
inary system has a
1
's place, a
2
's place,
a
4
's place, an
8
's place, a
16
's place and so on. We say the number places are grouped by
2's because
multiplying each number place by 2 gives you the next number place
. So:
1x2=2 (the 2's place), 2x2=4 (the 4's
place), 4x2=8 (the 8's place), 8x2=16 (the 16's place)
etc.
Let's look at the decimal number Let's look at the decimal number
103
in
binary format
:
01100111
<

read from right to left
We have a
1
in the
1's place
We have a
1
in the
2's place
We have a
1
in the
4's place
We have a
0
in the
8's place
We have a
0
in the
16's place
We have a
1
in the
32's place
We have a
1
in the
64's place
5
We have a
0
in the
128's place
Thus:
0+64+32+0+0+4+2+1=103
Okay, Let's test your skills. Here is a list of binary nu
mbers, try converting them to decimal
and check your answers at the end of this post.
10000000
11000000
11100000
01000000
10000011
10010001
11111111
If you were able to convert these numbers to decimal then congratulations! You're ready to
move on
to the next section.
[Understanding a subnet mask]
Now that you understand what binary is, let's have a look at our two subnet masks from the
beginning of my post:
192.168.1.0 / 255.255.255.0
192.168.1.0/24
The concept of a subnet mask is simple. Yo
u have a network and you have hosts on the
network (anything with an IP address is a host).
The subnet mask determines what
portion of the TCP/IP address represents your network and what portion can be
used for your hosts
. Because I am a simple person, I t
hink of it like this; The network
number represents the street I live on, and the host portion is used for the numbers on all the
houses on my street.
A subnet mask of
255.255.255.0
means that the first
three
octets of the address will be used
for the ne
twork, and thus our network number is
192.168.1
. This means we can have
254
computers on this network, because the fourth octet is not being used by the network portion
of the address. We know this because of the
0
in the subnet mask (255.255.255.
0
).
We
call each of the number sections an
octet
because we think of them in binary, and there
are eight possible bits in each section. Eight bits is an octet.
11111111
in binary is
255
in
decimal (did you do the conversions?). So our decimal subnet mask 255.255.
255.0 displayed
in binary is going to be:
11111111.11111111.11111111.00000000
If you count all the ones, you will find that there are
24
of them. Now look at the subnet mask
examples again.
192.168.1.0/255.255.255.0
192.168.1.0/24
Do you see why
both
subnet masks are the same
? The number
24
is the number of
bits
used
in the network portion of the address, and is short

hand for writing the address/subnet mask
combination. It becomes important to understand this when you start dividing your network
into
multiple sub networks.
6
[Understanding Subnetting]
Before reading this section, you should have a
good understanding
of what a subnet mask is
and how binary bits represent the subnet mask.
Simply put, subnetting is
dividing your network into
multiple s
ub networks
. To go back to
my silly example about houses and streets, subnetting gives you multiple streets in your
neighborhood.
There are
two methods
for dividing your network into multiple sub networks; One is to simply
change your network numbers keep
ing the same subnet mask. The other is to subnet your
network into smaller sub networks.
Keeping the same mask:
Your network could be divided into two or more networks by changing the network portion of
the address such as
192.168.1
and
192.168.2
and keep
ing the same subnet mask.
Example:
192.168.1.0/255.255.255.0
192.168.2.0/255.255.255.0
Doing this would give you
two separate networks
with
254 hosts per network
. This is a
very common method of dealing with multiple networks. However, back in the good o
ld days
you had to pay for every IP address you used, and if you had 25 computers on your network
you probably would not want to pay for 254 addresses! The answer to the problem
is...subnetting.
Subnetting a network:
Subnetting is when you use bits from t
he host portion of your address as part of
your network number
. This let's you subdivide your network at the cost of host addresses,
which is great if you're paying for every host IP address. It will save you money because you
pay for fewer TCP/IP addresse
s. Confused? Here is where understanding binary is important.
Lets look at a new subnet mask:
255.255.255.224
As you can see in the fourth octet, some of the host portion of this subnet mask is now being
used for part of the network address. Which means
we are
now using some of the binary
bits in the fourth octet for our network numbers
, and that gives us fewer hosts than our
old mask (which gave us 254), but gives us more networks (which is why we call it
subnetting).
How can we tell how many networks
and hosts per network this new subnet mask will give us?
Well... we shall have to use some of our newly acquired binary skills.
The
first task
is to find out
how many bits in the fourth octet are being used
? The
decimal number is 224, what is the decimal
number 224 as represented in binary?
The decimal number
224
in binary is:
11100000
We have a
0
in the
1's place
We have a
0
in the
2's place
We have a
0
in the
4's place
We have a
0
in the
8's place
We have a
0
in the
16's place
We have a
1
in the
32's
place
We have a
1
in the
64's place
7
We have a
1
in the
128's place
Thus: 128+64+32+0+0+0+0+0=
224
So our complete subnet mask in binary is:
1111111.11111111.11111111.
11100000
We now know that three bits from the fourth octet are used. How can we tell ho
w many sub
networks we're going to have? This requires some math

sorry. The formula is:
2
n

2
, where
n
is the
number of bits being used from the host portion
of our subnet mask.
Note:
We
subtract 2 from the total
because you do not count all 0's or all 1
's.
The formula for
three bits
is:
2
3

2
=6
In simpler terms:
(2x2x2)

2
=6
So our network is
sub divided into 6 networks
. Next, we want to know what the network
numbers are, and how many hosts we can have on each of the 6 networks?
What is the first subne
t? Let's have a look at the bits in our
fourth octet
again. The bit that
gives us the answer is the
(1) closest to the first zero
, and in this case it is the 3rd bit from
the left.
11
1
00000
The 3rd bit will
start our first network
, and the 3rd bit is in
the
32
's place (remember binary).
Start adding the value 32 to itself six times to get the six network numbers.
Note:
A quicker way to find our starting network number is to
subtract our mask from 256
.
256

224
=32
Here are our network numbers:
32
64
96
1
28
160
192
A better way to display this is:
192.168.1.
32
192.168.1.
64
192.168.1.
96
192.168.1.
128
192.168.1.
160
192.168.1.
192
The host addresses will
fall between the network numbers
, so we will have
30
hosts per
network. You're probably wondering why i
t's
not
31? The answer is that the last address of
each subnet is used as the
broadcast address
for that subnet.
Example:
Subnet:
192.168.1.32 / 255.255.255.224
8
Address Range:
192.168.1.33 through 192.168.1.62 (30 hosts)
Subnet Broadcast Address:
192.168.1
.63
Quiz:
Let's test your skills

write the address range and broadcast address for the following subnet.
You will find the answer at the end of this post.
Subnet:
192.168.1.128 / 255.255.255.224
Address Range
?
Subnet Broadcast Address
?
If we we're pa
ying for our TCP/IP addresses, we would only pay for one network and host
combination, thus paying for 30 hosts and
not
254. It could mean some real savings, it also
frees up the remaining addresses for other organizations to use.
Let's look at another su
bnet mask:
255.255.255.240
How many bits are used from the host portion? To find this out, we need to know how the
decimal number 240 is represented in binary.
The answer is:
11110000
So four bits are taken from the host portion of our mask. We do the
same math as before:
2
4

2
=14
In simpler terms:
(2x2x2x2)

2
=14
We will have
14 sub networks
, and what will the network numbers be? Look at the
fourth
bit
, it's in the 16's place:
111
1
0000
Note:
A quicker way to find our starting network number is to
su
btract the value of our mask
from 256
. So:
256

240
=16
Start adding 16 to itself

fourteen times to get all 14 network numbers:
16
32
48
64
80
96
112
128
144
160
176
192
208
224
A better way to display our subnets is:
192.168.1.16
9
192.168.1.32
192.168.1
.48
192.168.1.64
192.168.1.80
192.168.1.96
192.168.1.112
192.168.1.128
192.168.1.144
192.168.1.160
192.168.1.176
192.168.1.192
192.168.1.208
192.168.1.224
The host addresses fall between the network numbers. So we will have 14 host addresses on
each of ou
r 14 sub networks (
remember
: the last or 15th address is the broadcast address for
that subnet).
If you had a small company with 10 hosts and needed to have a static IP address for all of
your hosts, you would be assigned a network/subnet mask and a valid
IP address range.
Here is an example of what that might look like:
Network
: 205.112.10.16/.255.255.255.240
Address Range
: 205.112.10.17 through 205.112.10.30
Subnet Broadcast Address
: 205.112.10.31
[Answers to Binary Conversions]
10000000 = 128
1100
0000 = 192
11100000 = 224
01000000 = 64
10000011 = 131
10010001 = 145
11111111 = 255
[Answer to Subnet Question]
Subnet:
192.168.1.128 / 255.255.255.224
Address Range:
192.168.1.129 through 192.168.1.158
Subnet Broadcast Address:
192.168.1.159
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