SUBNETTING
- Subnetting Explained with Examples
This tutorial explains Subnetting in easy language with
examples. This tutorial is divided in three sections. First section provides a
basic overview of Subnetting tools. Second section explains Subnetting process
in easy steps. Last section includes Subnetting examples for beginners.
Subnetting
Subnetting is a process of dividing
large network into the smaller networks based on layer 3 IP address. Every
computer in network has an IP address which represents its location in network.
There are two versions of IP address; IPv4 and IPv6. This tutorial explains
Subnetting in IPv4.
This tutorial is the second part of our
article “Network Addressing Explained with Subnetting and VLSM”. You can read
other parts of this article here.
This tutorial is the first part of this
article. It explains how computers find each other in network with basic
terminology of network addressing.
This
tutorial is the last part of this article. It explains VLSM in detail with
examples including a unique six steps method of VLSM that will help you in
learning VLSM rapidly.
IPv4
IP addresses are displayed in dotted
decimal notation, and appear as four numbers separated by dots. Each number of
an IP address is made from eight individual bits known as octet. Each octet can
create number value from 0 to 255. An IP address would be 32 bits long in
binary divided into the two components, network component and host component.
Network component is used to identify the network that the packet is intend
for, and host component is used to identify the individual host on network.
IP addresses are broken into the two
components:
Network component :- Defines network
segment of device.
Host component :- Defines the
specific device on a particular network segment
IP Classes in decimal
notation
Class A addresses range from 1-126
Class B addresses range from 128-191
Class C addresses range from 192-223
Class D addresses range from 224-239
Class E addresses range from 240-254
·
0
[Zero] is reserved and represents all IP addresses.
·
127
is a reserved address and is used for testing, like a loop back on an
interface.
·
255
is a reserved address and is used for broadcasting purposes.
Subnet mask
Subnet mask is a 32 bits long address
used to distinguish between network address and host address in IP address.
Subnet mask is always used with IP address. Subnet mask has only one purpose,
to identify which part of an IP address is network address and which part is
host address.
For example how will we figure out
network partition and host partition from IP address 192.168.1.10 ? Here we
need subnet mask to get details about network address and host address.
·
In
decimal notation subnet mask value 1 to 255 represent network address and value
0 [Zero] represent host address.
·
In
binary notation subnet mask ON bit [ 1]
represent network address while OFF bit[0] represent
host address.
In decimal notation
IP address 192.168.1.10
Network address is 192.168.1 and host address
is 10.
In binary notation
IP address
11000000.10101000.00000001.00001010
Subnet mask
11111111.11111111.11111111.00000000
Network address is
11000000.10101000.00000001 and host address is 00001010
IP Class
|
Default Subnet
|
Network bits
|
Host bits
|
Total hosts
|
Valid hosts
|
A
|
255.0.0.0
|
First 8 bits
|
Last 24 bits
|
16, 777, 216
|
16, 777, 214
|
B
|
255.255.0.0
|
First 16 bits
|
Last 16 bits
|
65,536
|
65,534
|
C
|
255.255.255.0
|
First 24 bits
|
Last 8 bits
|
256
|
254
|
Network ID
First address of subnet is called
network ID. This address is used to identify one segment or broadcast domain
from all the other segments in the network.
Block Size
Block size is the size of subnet
including network address, hosts addresses and broadcast address.
Broadcast ID
There are two types of broadcast, direct
broadcast and full broadcast.
Direct broadcast or
local broadcast is the last address of subnet and
can be hear by all hosts in subnet.
Full broadcast is the last
address of IP classes and can be hear by all IP hosts in network. Full
broadcast address is 255.255.255.255
The main difference between direct
broadcast and full broadcast is that routers will not propagate local
broadcasts between segments, but they will propagate directed broadcasts.
Host Addresses
All address between the network address
and the directed broadcast address is called host address for the subnet. You
can assign host addresses to any IP devices such as PCs, servers, routers, and
switches.
Subnetting
Subnetting is a process of breaking
large network in small networks known as subnets. Subnetting happens when we
extend default boundary of subnet mask. Basically we borrow host bits to create
networks. Let's take a example
Being a network administrator you are
asked to create two networks, each will host 30 systems.
Single class C IP range can fulfill this
requirement, still you have to purchase 2 class C IP range, one for each
network. Single class C range provides 256 total addresses and we need only 30
addresses, this will waste 226 addresses. These unused addresses would make
additional route advertisements slowing down the network.
With subnetting you only need to
purchase single range of class C. You can configure router to take first 26
bits instead of default 24 bits as network bits. In this case we would extend
default boundary of subnet mask and borrow 2 host bits to create networks. By
taking two bits from the host range and counting them as network bits, we can
create two new subnets, and assign hosts them. As long as the two new network
bits match in the address, they belong to the same network. You can change
either of the two bits, and you would be in a new subnet.
Advantage of Subnetting
·
Subnetting
breaks large network in smaller networks and smaller networks are easier to
manage.
·
Subnetting
reduces network traffic by removing collision and broadcast traffic, that
overall improve performance.
·
Subnetting
allows you to apply network security polices at the interconnection between
subnets.
·
Subnetting
allows you to save money by reducing requirement for IP range.
Subnetting math
Subnetting process involves binary math
calculation. Computers communicate with each other's in binary language. To
succeed in any kind of networking career, you might be fluent in binary math
calculation. Subnetting needs two type of calculation, convert decimal to binary
and convert binary to decimal.
Base position
|
27
|
26
|
25
|
24
|
23
|
22
|
21
|
20
|
Decimal value
|
128
|
64
|
32
|
16
|
8
|
4
|
2
|
1
|
Binary system works exactly same as
decimal system, except the base number. Base number is 2 in binary system and
10 in decimal system. To calculate decimal equivalent value of a binary number,
you have to replace base value 10 with 2. Binary numbers are displayed in
columns and each position in binary system has double value than the position
in right. From earlier section of this article you know that each number of an
IP address is made from eight individual bits known as octet. So you should
remember at least eight decimal equivalent value from binary position.
Convert decimal to
binary
To convert a decimal number in binary we
would use addition till number method. In this method we start adding from left
to get target value. If after adding right position value, sum is lower than
target number, keep adding, or if sum is greater than target number skip the
position value. Only the value of on bit [1] will be added in sum. Off bit [0]
has zero value. For example, convert decimal number 117 in binary.
Target decimal number 117
Move direction From Left
===========================> to Right
Base position
|
27
|
26
|
25
|
24
|
23
|
22
|
21
|
20
|
Decimal value
|
128
|
64
|
32
|
16
|
8
|
4
|
2
|
1
|
Bit status
|
0
|
1
|
1
|
1
|
0
|
1
|
0
|
1
|
Decimal value in
addition
|
0
|
64
|
32
|
16
|
0
|
4
|
0
|
1
|
Binary value of 117 is 01110101.
Decimal calculation
|
Bit in binary
|
128 is greater than 117
|
off the bit
|
0+64 = 64 is less than 117
|
on the bit
|
0+64+32 = 96 is less than 117
|
on the bit
|
0+64+32+16 = 112 is less than 117
|
on the bit
|
0+64+32+16+8 = 120 is greater than
117
|
off the bit
|
0+64+32+16+0+4 = 116 is less than
117
|
on the bit
|
0+64+32+16+0+4+2 = 118 is greater
than 117
|
off the bit
|
0+64+32+16+0+4+0+1 = 117 is
equivalent to 117
|
on the bit
|
Convert binary in
decimal
To convert a binary in decimal we will
follow above method in reverse mode. We will find the decimal value of on
binary bit position and add them. For example convert 10101010 binary number in
decimal.
Target binary number 10101010
Move direction From Left
===========================> to Right
Base position
|
27
|
26
|
25
|
24
|
23
|
22
|
21
|
20
|
Decimal value
|
128
|
64
|
32
|
16
|
8
|
4
|
2
|
1
|
Bit status
|
1
|
0
|
1
|
0
|
1
|
0
|
1
|
0
|
Decimal value in
addition
|
128
|
0
|
32
|
0
|
8
|
0
|
2
|
0
|
Decimal value of 10101010 is 170 [
128+0+32+0+8+0+2+0 ]
Binary bit
|
Decimal value
|
1 On bit
|
128
|
0 Off bit
|
0
|
1 On bit
|
64
|
0 Off bit
|
0
|
1 On bit
|
32
|
0 Off bit
|
0
|
1 On bit
|
8
|
0 Off bit
|
0
|
1 On bit
|
2
|
0 Off bit
|
0
|
Review above method and do as much
practice of following as you can.
·
Pick
any number from 0 - 255 and convert it in binary.
·
Pick
any combination from 00000000 - 11111111 and convert it in decimal.
Better you are with this conversion, the
better you will do on the subnetting.
Combination provided by
binary position
Now you know the process of converting
binary in decimal and decimal in binary. Our next step is to figure out the
combination provided by a binary position.
First bit provide two combination 0 or
1. If we take two bits than we have four combinations 00,01,10,11. IP address
has 32 bits, so you should be able to find the number of combinations provided
by each bit position until position number 32.
Number of bit[s]
|
Number of combinations
|
Number of bits
|
Number of combinations
|
1
|
2
|
17
|
131072
|
2
|
4
|
18
|
262144
|
3
|
8
|
19
|
524288
|
4
|
16
|
20
|
1048576
|
5
|
32
|
21
|
2097152
|
6
|
64
|
22
|
4194304
|
7
|
128
|
23
|
8388608
|
8
|
256
|
24
|
16777216
|
9
|
512
|
25
|
33554432
|
10
|
1024
|
26
|
67108864
|
11
|
2048
|
27
|
134217728
|
12
|
4096
|
28
|
268435456
|
13
|
8192
|
29
|
536870912
|
14
|
16384
|
30
|
1073741824
|
15
|
32768
|
31
|
2147483648
|
16
|
65536
|
32
|
4294967296
|
Default subnet mask
Class
|
Subnet Mask
|
Format
|
A
|
255.0.0.0
|
Network.Host.Host.Host
|
B
|
255.255.0.0
|
Network.Network.Host.Host
|
C
|
255.255.255.0
|
Network.Network.Network.Host
|
Key terms to remembers
·
A
subnet is a smaller portion of large network treated as its own separate
network. To create subnet we borrow bits from host portion and assign them as
network bits. This mean more networks, fewer hosts.
·
If
the network bits on two addresses do not match, then the two packets are
intended for two separate networks.
·
On
a 32 bits IP address at least eight bits must belong to the network portion and
at least 2 bits must belong to the host portion.
·
Each
IP address has a predefined IP class and that cannot be changed.
·
Each
class has a predefined default subnet mask that tell us the octets, which are
already part of the network portion, as well as how many bits we have available
to work with.
·
Whatever
network class is it, we cannot change those bits that are already assigned.
·
We
cannot assign the network ID and the broadcast address to a host.
·
Regardless
how many bits are left in the host field, network ID and the broadcast address
must be reserved.
·
Subnet
bits start at the left and go to the right, without skipping bits.
CIDR [ Classless Inter Domain
Routing]
CIDR is a slash notation of subnet mask.
CIDR tells us number of on bits in a network address.
·
Class
A has default subnet mask 255.0.0.0. that means first octet of the subnet mask
has all on bits. In slash notation it would be written as /8, means address has
8 bits on.
·
Class
B has default subnet mask 255.255.0.0. that means first two octets of the
subnet mask have all on bits. In slash notation it would be written as /16,
means address has 16 bits on.
·
Class
C has default subnet mask 255.255.255.0. that means first three octets of the
subnet mask have all on bits. In slash notation it would be written as /24,
means address has 24 bits on.
Method of subnetting
In subnetting we find the answer of
following questions.
·
What
is subnet mask for given address?
·
How
many subnets does given subnet mask provide ?
·
What
is block size for given subnet mask?
·
What
are the valid subnets?
·
What
are the total hosts?
·
How
many valid hosts are available per subnet?
·
What
is broadcast address of each subnet?
·
What
is network address of each subnet?
To answer above questions we use
following method of subnetting.
What is subnet mask
for given address?
Subnetting take places when we extend
the default subnet mask. We cannot perform subnetting with default subnet mask
and every classes have default subnet mask. To figure out subnetted subnet mask
,we first need to write down the default subnet mask. Now find the host bits
borrowed to create subnets and convert them in decimal. For example find the
subnet mask of address 188.25.45.48/20 ? This address belong to class B and
class B has default subnet mask 255.255.0.0[ /16 in CIDR ]. We borrowed 4 bits
from hosts portion. As you know subnetting move from left to right and it
cannot skip any network bit. So this subnet mask in binary would be 11111111.
11111111.11110000.00000000. First two octet have default value so its decimal
value would be 255.255. We will convert third octet in decimal value. To
convert a binary number in decimal we add its decimal equivalent value. In our
example it would be 128+64+32+16+0+0+0+0 = 240. Our fourth octet has all bits
off so its decimal value would be 0+0+0+0+0+0+0+0 =0. Our answer subnet mask
would be 255.255.240.0
How many subnets does
given subnet mask provide ?
To calculate the number of subnets provided
by given subnet mask we use 2N , where N =
number of bits borrowed from host bits to create subnets. For example in
192.168.1.0/27, N is 3. By looking at address we can determined that this
address is belong to class C and class C has default subnet mask 255.255.255.0
[/24 in CIDR]. In given address we
borrowed 27 - 24 = 3 host bits to create
subnets. Now 23 = 8, so our answer is 8.
What is block size for
subnet mask?
Block size or increment number is used
to calculate the valid subnets. Once you figure out the block size, calculation
of valid subnets become piece of cake. To figure out the block size, use this
formula 256 - Subnet mask = block size. For example block size for subnet mask
255.255.255.240 is 256 - 240 = 16.
What are the valid
subnets?
Calculating valid subnet is two steps
process. First calculate total subnet by using formula 2N. In second step find
the block size and count from zero in block until you reach the subnet mask
value. For example calculate the valid subnets for 192.168.1.0/26.
Borrowed host bits are 2 [26-24].
Total subnets are 2
2 = 4.Subnet mask would be 255.255.255.192.Total subnets are 2
Block size would be 256-192 = 64.
Start counting from zero at blocks of 64, so our valid subnets would be 0,64,128,192.
What are the total
hosts?
Total hosts are the hosts available per
subnet. To calculate total hosts use formula 2H = Total hosts. H
is the number of host bits. For example in address 192.168.1.0/26 we have 32 -
26 [Total bits in IP address - Bits consumed by network address] = 6. Total
hosts per subnet would be 26 = 64.
How many valid hosts
are available per subnet?
Valid hosts are the number of hosts
those can be assigned to devices. As we know, we need to reduce two address per
subnet, one for network ID and another for broadcast ID. So our formula, to
calculate valid hosts would be Total hosts - 2 = Valid hosts. In above example
we have 64 hosts per subnet, so valid hosts in each subnet would be 64 - 2 =
62.
What is broadcast
address of each subnet?
Broadcast address is the last address of
subnet. This address is reserve for network broadcast, and cannot be assigned
to any host. In above example
0 Subnet has broadcast address 63
64 Subnet has broadcast address 127
128 Subnet has broadcast address 191
192 Subnet has broadcast address 255
64 Subnet has broadcast address 127
128 Subnet has broadcast address 191
192 Subnet has broadcast address 255
What is the network
address of each subnet?
Network address is the first address of
subnet. This address is used to locate the network, and cannot be assigned to
any host. In above example address 0,64,128,192 are the network address.
·
Network
address is always the first IP address of subnet.
·
Broadcast
address is always the last IP address of subnet ( IP address before the next
subnet).
·
Valid
hosts are the IP addresses between network address and broadcast address.
At this point you have powered with all
essential tools for subnetting. In last section of this article we will
practically implement what we have learn so far. Due to length of this article
I will include examples only from class C.
Class C Subnetting
Default subnet mask of class C is
255.255.255.0. CIDR notation of class C is /24, which means 24 bits from IP
address are already consumed by network portion and we have 8 host bits to work
with. We cannot skip network bit, when we turned them on. Subnetting moves from
left to right. So Class C subnet masks can only be the following:
CIDR
|
Decimal
|
Binary
|
/25
|
128
|
10000000
|
/26
|
192
|
11000000
|
/27
|
224
|
11100000
|
/28
|
240
|
11110000
|
/29
|
248
|
11111000
|
/30
|
252
|
11111100
|
As we have already discussed earlier in
this article that we have to have at least 2 host bits for assigning IP
addresses to hosts, that means we can't use /31 and /32 for subnetting.
/25
CIDR /25 has subnet mask 255.255.255.128
and 128 is 10000000 in binary. We used one host bit in network address.
N = 1 [Number of host bit used in
network]
H = 7 [Remaining host bits]
Total subnets ( 2
N ) :- 21 = 2Block size (256 - subnet mask) :- 256 - 128 = 128H = 7 [Remaining host bits]
Total subnets ( 2
Valid subnets ( Count blocks from 0) :- 0,128
Total hosts (2
H) :- 27 = 128Valid hosts per subnet ( Total host - 2 ) :- 128 - 2 = 126
Subnets
|
Subnet 1
|
Subnet 2
|
Network ID
|
0
|
128
|
First host
|
1
|
129
|
Last host
|
126
|
254
|
Broadcast ID
|
127
|
255
|
/26
CIDR /26 has subnet mask 255.255.255.192
and 192 is 11000000 in binary. We used two host bits in network address.
N = 2
H = 6
Total subnets ( 2
N ) :- 22 = 4Block size (256 - subnet mask) :- 256 - 192 = 64H = 6
Total subnets ( 2
Valid subnets ( Count blocks from 0) :- 0,64,128,192
Total hosts (2
H) :- 26 = 64Valid hosts per subnet ( Total host - 2 ) :- 64 - 2 = 62
Subnets
|
Subnet 1
|
Subnet 2
|
Subnet 3
|
Subnet 4
|
Network ID
|
0
|
64
|
128
|
192
|
First host
|
1
|
65
|
129
|
193
|
Last host
|
62
|
126
|
190
|
254
|
Broadcast ID
|
63
|
127
|
191
|
255
|
/27
CIDR /27 has subnet mask 255.255.255.224
and 224 is 11100000 in binary. We used three host bits in network address.
N = 3
H = 5
Total subnets ( 2
N ) :- 23 = 8Block size (256 - subnet mask) :- 256 - 224 = 32H = 5
Total subnets ( 2
Valid subnets ( Count blocks from 0) :- 0, 32, 64, 96, 128, 160, 192, 224
Total hosts (2
H) :- 25 = 32Valid hosts per subnet ( Total host - 2 ) :- 32 - 2 = 30
Subnets
|
Sub 1
|
Sub 2
|
Sub 3
|
Sub 4
|
Sub 5
|
Sub 6
|
Sub 7
|
Sub 8
|
Network ID
|
0
|
32
|
64
|
96
|
128
|
160
|
192
|
224
|
First host
|
1
|
33
|
65
|
97
|
129
|
161
|
193
|
225
|
Last host
|
30
|
62
|
94
|
126
|
158
|
190
|
222
|
254
|
Broadcast ID
|
31
|
63
|
95
|
127
|
159
|
191
|
223
|
255
|
Sub
= Subnet
/28
CIDR /28 has subnet mask 255.255.255.240
and 240 is 11110000 in binary. We used four host bits in network address.
N = 4
H = 4
Total subnets ( 2
N ) :- 24 = 16Block size (256 - subnet mask) :- 256 - 240 = 16H = 4
Total subnets ( 2
Valid subnets ( Count blocks from 0) :-0, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, 240
Total hosts (2
H) :- 24 = 16Valid hosts per subnet ( Total host - 2 ) :- 16 - 2 = 14
I hope you have understand the pattern
of making the subnet chart with above information's. Let's join me in this
practice. For this example and next two examples I will fill only two subnets
[first and last one], it's your turn to fill the middle subnets.
Subnets
|
Subnet 1
|
Subnet 2 To Subnet 15(Filled by
you)
|
Subnet 16
|
Network ID
|
0
|
........................................
|
240
|
First host
|
1
|
........................................
|
241
|
Last host
|
14
|
........................................
|
254
|
Broadcast ID
|
15
|
......................................
|
255
|
/29
CIDR /29 has subnet mask 255.255.255.248
and 248 is 11111000 in binary. We used five host bits in network address.
N = 5
H = 3
Total subnets ( 2
N ) :- 25 = 32Block size (256 - subnet mask) :- 256 - 248 = 8H = 3
Total subnets ( 2
Valid subnets ( Count blocks from 0) :-
0, 8, 16, 24, 32, 40, 48, 56, 64, 72, 80, 88, 96, 104, 112, 120, 128, 136, 144, 152, 160, 168, 176, 184, 192, 200, 208, 216, 224, 232, 240, 248
Total hosts (2
H) :- 23 = 8Valid hosts per subnet ( Total host - 2 ) :- 8 - 2 = 6
Subnets
|
Subnet 1
|
Subnet 2 To Subnet 31(Filled by
you)
|
Subnet 32
|
Network ID
|
0
|
..........................................
|
248
|
First host
|
1
|
..........................................
|
249
|
Last host
|
6
|
..........................................
|
254
|
Broadcast ID
|
7
|
.........................................
|
255
|
/30
CIDR /30 has subnet mask 255.255.255.252
and 252 is 11111100 in binary. We used six host bits in network address.
N = 6
H = 2
Total subnets ( 2
N ) :- 26 = 64Block size (256 - subnet mask) :- 256 - 252 = 4H = 2
Total subnets ( 2
Valid subnets ( Count blocks from 0) :-
0, 4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48, 52, 56, 60, 64, 68, 72, 76, 80, 84, 88, 92, 96, 100, 104, 108, 112, 116, 120, 124, 128, 132, 136, 140, 144, 148, 152, 156, 160, 164, 168, 172, 176, 180, 184, 188, 192, 196, 200, 204, 208, 212, 216, 220, 224, 228, 232, 236, 240,2 44, 248, 252
Total hosts (2
H) :- 22 = 4Valid hosts per subnet ( Total host - 2 ) :- 4 - 2 = 2
Subnets
|
Subnet 1
|
Subnet 2 To Subnet 63 (Filled by
you)
|
Subnet 64
|
Network ID
|
0
|
..........................................
|
252
|
First host
|
1
|
..........................................
|
253
|
Last host
|
2
|
..........................................
|
254
|
Broadcast ID
|
3
|
.........................................
|
255
|
At the end of this long article I have a
small word for you, that is practice. Believe me in exam you are going to face
many questions related to subnetting. More practice will make you more
comfortable in exam.
