Rahul Khadse
IP Subnet :-- In TCP/IP by default three sizes of networks are available:
-
In Class C
Example
Remaining Subnet
144 – 159
VLSM
(1) Class A -224 PC -> 16777216
(2) Class B - 216 PC-> 65536
(3) Class C – 28 PC -> 256
In subneting, we will divide class A,B & C network into
small size sub networks. This procedure is called subneting.Subneting
is performed with the help of subnet mask. There are two types of subneting
that we performed: -
- FLSM Fixed Length Subnet Mask
- VLSM Variable Length Subnet Mask
Why
to Sub?
(i) Default Class Network provide us large no. of PCs in
comparison to the requirement of PCs in
the network.
(ii) It is practical never possible to create a class A or
class B sized network.To reduce
the broadcast of network, we have to perform LAN segmentation of routers. In
each sub network, we need different network addresses.
How
to Subnet?
In this formula, we will first modify our requirement
according to the no. of subnet possible then we calculate new subnet mask and
create IP range.
Example 1
Class = C
No. of subnet =5
Step1
No. of subnet possible is 2,4,8,16,32……
Class= C
No. of subnets= 8
Step 2
Calculate key value
2? = No. of subnets
2? = 8
23= 8
Step 3
Calculate new subnet mask
In class C
Net id Host
id
24+key 8-key
24+3 8-3
27 5
11111111.11111111.11111111.11100000
255. 255. 255. 224
We add this address to make subnet mask
Step 4
Range
No. of Pc/Subnet= Total Pc/ No. of Subnet
= 256/8 =32
x.x.x.0 – x.x.x.31
(1)- (30)
x.x.x.32- x.x.x.63
64- 95
96- 127
128- 159
160- 191
192- 223
x.x.x.224-x.x.x.255
The first IP of each subnet will be subnet id and last IP
will be sub network broadcast address.
Example 2
Class= C
No. of subnet= 10
Step 1
No. of subnet= 16
Step 2
24= 16
Step 3
Net id
Host id
24+4
8-4
11111111.11111111.11111111.11110000
Subneting method 2
Class=
No. of Pc/Sub= 8
Mask= ?
Range= ?
In this case we have to calculate the key according to the
no. of per subnet according to the key value the bits of subnet mask from right
hand side are set to zero then range is calculated.
Class= C
No. of Pc/Sub=5
Step 1
No. of Pc/Subnet possible 4,8,16,32,64….
New requirement
Class= C
No. of Pc/Sub= 8
Step 2
2?= No. of Pc/Sub
2?= 8
23= 8
\
\/
key 3
11111111.11111111.11111111.11111000
255. 255. 255. 248
No. of Subnet= Total Pc/(Pc/Sub)
= 256/8
Class C Sub Pc/Sub
255.255.255.248 32 8
200.100.100.0
200.100.100.7
.8 .15
.16 .23
.24 .31
.
.
Example 2
Class C
No. of Pc/Sub=50
Step 1
Class= C
No. of Pc/Sub= 64
Step 2
Class C Sub Pc/Sub
No. of Pc/Sub= 50
Example: - Check whether an address is valid IP, N/w
address or Broadcast address. If IP is valid then calculate its N/w &
Broadcast address.
Step 2
26= 64
11111111.11111111.11111111.11000000
255. 255. 255. 192
No. of subnet= 256/64= 4
255.255.255.192
4 64
Method 3
New req.
No. of Pc/Sub= 64
No. of Subnet= 256/64= 4
Class= C
No. of Sub= 4
22= 4
24+2 8-2
11111111.11111111.11111111.11000000
255. 255. 255. 192
Zero Subnet :-- According to the rules of IP Addressing the first subnet and
last subnet is not useable due to routing problem. In new Cisco router a
command is present in default configuration. With this command, we are able to
use first and last Subnet after Subneting.
Command is
Router#config ter
Router(config)#ip subnet-zero
Router(config)#exit
200.100.100.197
255.255.255.240
28
4
200.100.100.197
200.100.100.1100 0101 --------> Valid IP
200.100.100.192
200.100.100.1100 0000 -------> Network Address
200.100.100.207
200.100.100.1100 1111 -------> Broadcast Address
Example: -
Class= B
No. of subnet= 64
26= 64
11111111.11111111.11111111.11000000
255. 255. 255. 192
No. of Pc/Sub= 65536/64= 1024
150.20.0.0 – 150.20.3.255
150.20.4.0 – 150.20.7.255
150.20.8.0 – 150.20.11.255
Prefix Notation of representing IP Address
IP address can be written as IP & Mask as well as
IP/Prefix.
200.100.100.18
255.255.255.248
200.100.100.18/29
170.20.6.6
255.255.224.0
170.20.6.6/19
This method is representing IP address also called CIDR
(Classless Inter Domain Routing) notation.
Remaining Subnet
144 – 159
160 – 175
176 – 191
192 – 207
208 – 223
224 – 239
240 – 255
Problem with FLSM
In FLSM, we have to create subnet of equal size. All N/w
will be allotted constant size subnet instead of their IP addresses
requirement. Due to this a N/w may be allotted more than required IP address
and less than required IP addresses.
VLSM
| \25 255.255.255.128 |
\26 255.255.255.192 |
\27 255.255.255.224 |
\28 255.255.255.240 |
\29 255.255.255.248 |
| Sub Pc/Sub 2 128 |
Sub Pc/Sub 4 64 |
Sub Pc/Sub 8 82 |
Sub Pc/Sub 16 16 |
Sub Pc/Sub
32 8 |
| 0 - 127 | 0 - 63 | 0 - 31 | 0 - 15 | 0 - 7 |
| 128 - 255 | 64 - 127 | 32 - 63 | 16 - 31 | 8 - 15 |
| 128 - 191 | 64 - 95 | 32 - 47 | 16 - 23 | |
| 192 - 255 | 96 - 127 | 48 - 63 | 24 - 31 | |
| 64 - 79 | ||||
| 80 - 95 | ||||
| 96 - 111 |
If we are using VLSM and Dynamic Routing then routing be
compatible to VLSM. This will happen only if Subnet masks are also sends in the
routing updates.
Super Netting :-- Combining small N/w to create a large size N/w is
called Super Network. Super netting is mostly used to define route
summarizations in routing tables. It is not used for the implementation of
large network.
170.10.0.0 170.00001010.00000000.00000000
170.11.0.0 1 70.00001011.00000000.00000000
Posted in: CCNA
