Subnetting is a logical procedure of
the deviding large no. of network into
smaller segment.
The two types of subnetting are:
- Static length subnet mask(fixed-length
subnet mask (FLSM))
- Variable length subnet mask (VLSM)
Static Length' subnetting
If all the subnetworks in a single network use the same subnet mask, it is called as 'Static Length' subnetting.
If all the subnetworks in a single network use the same subnet mask, it is called as 'Static Length' subnetting.
Ex: In FLSM If address starts with 195.3.2.1/26, then whole network
will belong to same subnetmask and we can't assign as another subnet
like 197.22.75.96/28
Variable
Lenght Subnet Mask (VLSM)
Variable Length Subnet Masking - VLSM - is a technique
that allows network administrators to divide an IP address into different subnets
sizes, unlike simple same-size Subnetting (FLSM).
The VLSM diagram below:
IP SUBNETTING:
Class c Subnetting: In
subnetting we are borrowing host bits to network bit.
Class c have 24 network bits
and 8 host bits so
Total no of network in class C= 221=2097152
Total Host per Network in class C= 28-2 =254
If we want to make more network which contain less than
254 host per network so we will do subnetting and we will borrow bits in
network from host.
Before starting we should understand subnet bits and
there value:
Host
Octect
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1 2 3 4 5 6 7 8
128 64 32 16 8 4 2 1
27 + 26 + 25
+ 24 + 23 + 22 + 21
+ 20
128 for One bit borrow
|
27=128
|
64 for Two bit borrow
|
26=64
|
32 for Three bit borrow
|
25=32
|
16 for four bit borrow
|
24=16
|
8 for five bit borrow
|
23=8
|
4 for six bit borrow
|
22=4
|
2 for seven bit borrow
|
21=2
|
1 for eight bit borrow
|
20=1
|
Example:
192.168.10.0/27
Total no of network bit =24+3=27 (class c have
24 network bit and added borrowed three bits)
Here we have borrowed three bit from host to
network so subnet will be
128+64+32=224
255.255.255.224
Maximum No of subnet= 23=8 (Borrow
bit Three)
Total no of host bit=8 (Host bit) - 3 (Borrowed Bits) = 5 (class c
have 8 host bit and three bit borrowed so we have minus)
Maximum no of host per subnet= 25-2=30
Block Size=256-224=32
Subnetting:
Subnet ,
Valid Hosts , Broadcast
1- 192.168.10.0 ,
192.168.10.1 to 192.168.10.30 , 192.168.10.31
2- 192.168.10.32 ,
192.168.10.33 to 192.168.10.62 , 192.168.10.63
3- 192.168.10.64 ,
192.168.10.65 to 192.168.10.94 , 192.168.10.95
4- 192.168.10.96 ,
192.168.10.97 to 192.168.10.126 , 192.168.10.127
5- 192.168.10.128 ,
192.168.10.129 to 192.168.10.158 , 192.168.10.159
6- 192.168.10.160 ,
192.168.10.161 to 192.168.10.190
,192.168.10.191
7- 192.168.10.192 ,
192.168.10.193 to 192.168.10.222
,192.168.10.223
8- 192.168.10.224 ,
192.168.10.225 to 192.168.10.254
,192.168.10.255
Class B Subnetting: In
subnetting we are borrowing host bit to network bit.
Class B have 16 network bits
and 16 host bits so
Total no of network in class B= 214=16384
Total Host per Network in class B= 216-2
=65534
If we want to make more network which contain less than
254 host per network so we will do subnetting and we will borrow bits in
network from host.
Before starting we should understand subnet bits and
there value:
Host
Octect
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1 2 3 4 5 6 7 8
128 64 32 16 8 4 2 1
27 + 26 + 25
+ 24 + 23 + 22 + 21
+ 20
128 for One bit borrow
|
27=128
|
64 for Two bit borrow
|
26=64
|
32 for Three bit borrow
|
25=32
|
16 for four bit borrow
|
24=16
|
8 for five bit borrow
|
23=8
|
4 for six bit borrow
|
22=4
|
2 for seven bit borrow
|
21=2
|
1 for eight bit borrow
|
20=1
|
Example:
172.16.10.0/18
Total no of network bit =16+2=18 (class B have 16
network bit and added borrowed two bits)
Here we have borrowed three bit from host to
network so subnet will be
128+64=192
255.255.192.0
Maximum No of subnet= 22=4 (Borrow
bit Two)
Total no of host bit=16 (Host bit) - 2 (Borrowed Bits) = 14 (class B
have 16 host bit and two bit borrowed so we have minus)
Maximum no of host per subnet= 214-2=16382
Block Size=256-192=64
Subnetting:
Subnet ,
Valid Hosts , Broadcast
1- 172.160.0.0 ,
172.160.0.1 to 172.160.63.254
, 172.160.63.255
2- 172.160.64.0 ,
172.160.64.1 to 172.160.127.254
, 172.160.127.255
3- 172.160.128.0 ,
172.160.128.1 to 172.160.191.254 ,172.160.191.255
4- 172.160.192.0 , 172.160.192.1 to 172.160.255.254 ,172.160.255.255
Subnetting with more than 8 bits:
Example:
172.16.10.0/27
Total no of network bit =16+11=27 (class B have
16 network bit and added borrowed 11 bits)
Here we have borrowed 11 bit from host to
network so subnet will be
128 +64+32+16+8+4+2+1=
255 128+64+32=224
255.255.255.224
Maximum No of subnet= 211=2048
(Borrow bit Three)
Total no of host bit=16 (Host bit) - 11 (Borrowed Bits) = 5 (class B
have 16 host bit and 11 bit borrowed so we have minus)
Maximum no of host per subnet= 25-2=30
Block Size=256-224=32
Subnetting:
Subnet
, Valid Hosts , Broadcast
172.16.0.0
, 172.16.0.1 to 172.16.0.30 ,
172.16.0.31
172.16.0.32
, 172.16.0.33 to 172.16.0.62 ,
172.16.0.63
172.16.0.64
, 172.16.0.65 to
172.16.0.94 , 172.16.0.95
172.16.0.96
, 172.16.0.97 to
172.16.0.126 , 172.16.0.127
172.16.0.128
, 172.16.0.129 to 172.16.0.158 ,
172.16.0.159
172.16.0.160
, 172.16.0.161 to
172.16.0.190 , 172.16.0.191
172.16.0.192
, 172.16.0.193 to
172.16.0.222 , 172.16.0.223
172.16.0.224
, 172.16.0.225 to
172.16.0.254 , 172.16.0.255
172.16.1.0
, 172.16.1.1 to 172.16.1.30 ,
172.16.1.31
172.16.1.32
, 172.16.1.33 to
172.16.1.62 , 172.16.1.63
172.16.1.64
, 172.16.1.65 to
172.16.1.94 , 172.16.1.95
172.16.1.96
, 172.16.1.97 to
172.16.1.126 , 172.16.1.127
172.16.1.128
, 172.16.1.129 to
172.16.1.158 , 172.16.1.159
172.16.1.160
, 172.16.1.161 to
172.16.1.190 , 172.16.1.191
172.16.1.192
, 172.16.1.193 to
172.16.1.222 , 172.16.1.223
172.16.1.224
, 172.16.1.225 to
172.16.1.254 , 172.16.1.255
172.16.2.0
, 172.16.2.1 to 172.16.2.30 ,
172.16.2.31
172.16.2.32
, 172.16.2.33 to
172.16.2.62 , 172.16.2.63
172.16.2.64
, 172.16.2.65 to 172.16.2.94 ,
172.16.2.95
172.16.2.96
, 172.16.2.97 to
172.16.2.126 , 172.16.2.127
----- -------------- ------------
172.16.255.224 , 172.16.255.225 to 172.16.255.254 , 172.16.255.255
Class A Subnetting: In
subnetting we are borrowing host bit to network bit.
Class A have 8 network bits
and 24 host bits so
Total no of network in class B= 27
=128
Total Host per Network in class B= 224-2 =16777214
If we want to make more network which contain less than
254 host per network so we will do subnetting and we will borrow bits in
network from host.
Before starting we should understand subnet bits and
there value:
Host
Octect
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1 2 3 4 5 6 7 8
128 64 32 16 8 4 2 1
27 + 26 + 25
+ 24 + 23 + 22 + 21
+ 20
128 for One bit borrow
|
27=128
|
64 for Two bit borrow
|
26=64
|
32 for Three bit borrow
|
25=32
|
16 for four bit borrow
|
24=16
|
8 for five bit borrow
|
23=8
|
4 for six bit borrow
|
22=4
|
2 for seven bit borrow
|
21=2
|
1 for eight bit borrow
|
20=1
|
Example:
112.0.0.0/12
Total no of network bit =8+4=12 (class A have 8
network bit and added borrowed 4 bits)
Here we have borrowed 4 bit from host to
network so subnet will be
128+64+32+16=240
255.240.0.0
Maximum No of subnet= 24=16 (Borrow
bit Three)
Total no of host bit=24 (Host bit) - 4 (Borrowed Bits) = 20 (class A
have 24 host bit and 4 bit borrowed so we have minus)
Maximum no of host per subnet= 220-2=1048574
Block Size=256-240=16
Subnetting:
Subnet
, Valid Hosts , Broadcast
112.0.0.0
, 112.0.0.1 to 112.15.255.254 ,
112.15.255.255
112.16.0.0
, 112.16.0.1 to
112.31.255.254 , 112.31.255.255
112.32.0.0
, 112.32.0.1 to
112.47.255.254 , 112.47.255.255
112.48.0.0
, 112.48.0.1 to
112.63.255.254 , 112.63.255.255
112.64.0.0
, 112.64.0.1 to
112.79.255.254 , 112.79.255.255
112.80.0.0
, 112.80.0.1 to
112.95.255.254 , 112.95.255.255
112.96.0.0
, 112.96.0.1 to
112.111.255.254 , 112.111.255.255
112.112.0.0
, 112.112.0.1 to
112.127.255.254 , 112.127.255.255
112.128.0.0
, 112.128.0.1 to
112.143.255.254 , 112.143.255.255
112.144.0.0
, 112.144.0.1 to
112.159.255.254 , 112.159.255.255
112.160.0.0
, 112.160.0.1 to
112.175.255.254 ,
112.175.255.255
112.176.0.0
, 112.176.0.1 to
112.191.255.254 , 112.191.255.255
112.192.0.0
, 112.192.0.1 to
112.207.255.254 , 112.207.255.255
112.208.0.0
, 112.208.0.1 to
112.223.255.254 , 112.223.255.255
112.224.0.0
, 112.224.0.1 to
112.239.255.254 , 112.239.255.255
112.240.0.0
, 112.240.0.1 to
112.255.255.254 , 112.255.255.255
Subnetting More than 8 bit:
Example:
112.0.0.0/26
Total no of network bit =8+18=26 (class A have
8 network bit and added borrowed 18 bits)
Here we have borrowed 18 bit from host to
network so subnet will be
128 +64+32+16+8+4+2+1=
255 128 +64+32+16+8+4+2+1= 255
128 +64
=192
255.255.255.192
Maximum No of subnet= 218=262144(Borrow
bit Three)
Total no of host bit=24 (Host bit) - 18 (Borrowed Bits) = 6 (class A
have 24 host bit and 18 bit borrowed so we have minus)
Maximum no of host per subnet= 26-2=62
Block Size=256-192=64
Subnetting:
Subnet
, Valid Hosts , Broadcast
112.0.0.0
, 112.0.0.1 to 112.0.0.62 ,
112.0.0.63
112.0.0.64
, 112.0.0.65 to 112.0.0.126 ,
112.0.0.127
112.0.0.128
, 112.0.0.129 to
112.0.0.190 , 112.0.0.191
112.0.0.192
, 112.0.0.193 to
112.0.0.254 , 112.0.0.255
112.0.1.0
, 112.0.1.1 to 112.0.1.62 ,
112.0.1.63
112.0.1.64
, 112.0.1.65 to 112.0.1.126 ,
112.0.1.127
112.0.1.128
, 112.0.1.129 to
112.0.1.190 , 112.0.1.191
112.0.1.192
, 112.0.1.193 to
112.0.1.254 , 112.0.1.255
112.0.2.0
, 112.0.2.1 to 112.0.2.62 ,
112.0.2.63
112.0.2.64
, 112.0.2.65 to 112.0.2.126 ,
112.0.2.127
112.0.2.128
, 112.0.2.129 to
112.0.2.190 , 112.0.2.191
112.0.2.192
, 112.0.2.193 to
112.0.2.254 , 112.0.2.255
112.0.3.0
, 112.0.3.1 to 112.0.3.62 ,
112.0.3.63
112.0.3.64
, 112.0.3.65 to 112.0.3.126 ,
112.0.3.127
112.0.3.128
, 112.0.3.129 to
112.0.3.190 , 112.0.3.191
112.0.3.192
, 112.0.3.193 to
112.0.3.254 , 112.0.3.255
112.0.4.0
, 112.0.4.1 to 112.0.4.62 ,
112.0.4.63
112.0.4.64
, 112.0.4.65 to 112.0.4.126 ,
112.0.4.127
112.0.4.128
, 112.0.4.129 to
112.0.4.190 , 112.0.4.191
112.0.4.192
, 112.0.4.193 to
112.0.4.254 , 112.0.4.255
-------------- ----------------------- ------------
112.255.255.192 112.255.255.193 to 112.255.255.254 112.255.255.255
Subnet zero: If a network address is subnetted,
the first subnet obtained after subnetting the network address is called subnet
zero.
Consider a
Class B address, 172.16.0.0. By default the Class B address 172.16.0.0 has 16
bits reserved for representing
the host portion, thus allowing 65534 (216−2) valid host addresses. If network
172.16.0.0/16 is subnetted
by borrowing three bits from the host portion, eight (23) subnets are obtained.
The table below is an example showing the subnets obtained by subnetting the
address 172.16.0.0, the resulting subnet mask, the corresponding broadcast
addresses, and the range of valid host addresses.
In the
example above, the first subnet (subnet 172.16.0.0/19) is called subnet zero.
The class
of the network subnetted and the number of subnets obtained after subnetting
have no role in
determining
subnet zero. It is the first subnet obtained when subnetting the network
address. Also, when you
write the
binary equivalent of the subnet zero address, all the subnet bits (bits 17, 18,
and 19 in this case) are
zeros.
Subnet zero is also known as the all−zeros subnet.
The
All−Ones Subnet
When a
network address is subnetted, the last subnet obtained is called the all−ones
subnet.
With
reference to the example above, the last subnet obtained when subnetting network
172.16.0.0 (subnet
172.16.224.0/19)
is called the all−ones subnet.
The class
of the network subnetted and the number of subnets obtained after subnetting
have no role in
determining
the all−ones subnet. Also, when you write the binary equivalent of the subnet
zero address, all the subnet bits (bits 17, 18, and 19 in this case) are ones,
hence the name
Problems
with Subnet Zero and the All−Ones Subnet
Traditionally,
it was strongly recommended that subnet zero and the all−ones subnet not be
used for
addressing.
According to RFC 950 , "It is useful to preserve and extend the
interpretation of these special
(network
and broadcast) addresses in subnetted networks. This means the values of all
zeros and all ones in
the subnet
field should not be assigned to actual (physical) subnets." This is the
reason why network engineers required
to calculate the number of subnets obtained by borrowing three bits would
calculate 23−2 (6) and not 23 (8). The −2 takes into account that subnet zero
and the all−ones subnet are not used traditionally.
Subnet−Zero
Using
subnet zero for addressing was discouraged because of the confusion inherent in
having a network and
a subnet
with indistinguishable addresses. With reference to our example above, consider
the IP address 172.16.1.10. If you calculate the subnet address corresponding
to this IP address, the answer you arrive at is subnet 172.16.0.0 (subnet
zero). Note that this subnet address is identical to network address
172.16.0.0, which was subnetted in the first place, so whenever you perform
subnetting, you get a network and a subnet (subnet zero) with indistinguishable
addresses. This was formerly a source of great confusion.
The
All−Ones Subnet
Use of the
all−ones subnet for addressing has been discouraged in the past because of the
confusion inherent
in having
a network and a subnet with identical broadcast addresses.
With
reference to the example above, the broadcast address for the last subnet
(subnet 172.16.224.0/19) is
172.16.255.255,
which is identical to the broadcast address of the network 172.16.0.0, which
was subnetted in the first
place, so whenever you perform subnetting you get a network and a subnet
(all−ones subnet) with identical
broadcast addresses. In other words, a network engineer could configure the
address 172.16.230.1/19 on a router, but if that is done, he can no longer
differentiate between a local subnet broadcast (172.16.255.255 (/19)) and the
complete Class B broadcast (172.16.255.255(/16)).
Using
Subnet Zero and the All−Ones Subnet
It should
be noted that even though it was discouraged, the entire address space including
subnet zero and the all−ones subnet have always been usable. The use of the
all−ones subnet was explicitly allowed and the use of subnet zero is explicitly
allowed since Cisco IOS Software Release 12.0. Even prior to Cisco IOS Software Release
12.0, subnet zero could be used by entering the ip subnet−zero global
configuration command. On the
issue of using subnet zero and the all−ones subnet, RFC 1878 states, "This
practice (of excluding all−zeros
and all−ones subnets) is obsolete. Modern software will be able to utilize all
definable networks." Today, the
use of subnet zero and the all−ones subnet is generally accepted and most
vendors support their use.
However, on certain networks, particularly the ones using legacy software, the
use of subnet zero and the all−ones subnet can lead to problems.
What is Broadcast
Address: A broadcast address is a logical address at which all devices connected to a multiple-access communications network are enabled to receive datagrams. A message sent to a broadcast address is typically received by all
network-attached hosts, rather than by a specific host.