1. Subnetting and Variable Length
Subnet Masks (VLSMs)

2. Understanding Subnetting
• Reduction of network traffic
• Optimization of network
performance
• Easy management
• Easy spanning in large geographical
distances

3. Understanding Subnetting
Class Format Default Subnet Mask
A network.node.node.node 255.0.0.0
B network.network.node.node 255.255.0.0
C network.network.network.node 255.255.255.0
IP Address : 32 Bits
From 00000000.00000000.00000000.00000000
To 11111111.11111111.11111111.11111111
In Hexadecimal:
From 00.00.00.00
To FF.FF.FF.FF (broadcasting)

4. Understanding Subnetting
Class Network Host Number of Addresses per Start End
Bits Bits Networks Network Address Address
A 8 24 128 (27) 16,777,216 (224) 0.0.0.0 127.255.
255.255
B 16 16 16,384 (214) 65,536 (216) 128.0.0.0 191.255.
255.255
C 24 8 2,097,152 (221) 256 (28) 192.0.0.0 223.255.
255.255

5. Easy way to subnet
A. Remember the Classless Inter-Domain
Routing table values, or
B. Apply the following system:
1. Observe the Example 192.168.1.23/26 (CIDR)
1. 255 is 8 bits
2. 255.255 is 16 bits
3. 255.255.255 is 24 bits
4. Then 255.255.255.?
5. We need 2 more bits !

6. Easy way to subnet
We need 2 bits to make /26
Octets Bit Needed
128 
192 
224
240
248
252
254
255
Subnet mask is 255.255.255.192

7. So, how many networks then?
128 64 32 16 8 4 2 1
 
Since 192.168.1.23 is a Class C network, we can only have 256 addresses
If we want to have 64 subnets, we need to use increments of 64 or:

8. So, how many networks then?
Subnet First Host Last Host Broadcasting
192.168.1.0 192.168.1.1 192.168.1.62 192.68.1.63
192.168.1.64 192.168.1.65 192.168.1.126 192.168.1.127
192.168.1.128 192.168.1.129 192.168.1.190 192.168.1.191
192.168.1.192 192.168.1.193 192.168.1.254 192.168.1.255

9. Variable Length Subnet Mask
VLSM
• VLSM designs use different subnet mask.
Example:
– Subnet 1: 192.168.10.64/29 (6 hosts)
– Subnet 2: 192.168.10.48/28 (10 hosts)
– Subnet 3: 192.168.10.72/30 (2 hosts)
• Makes the usage of IP addresses even more
efficient than standard Subnetting.

10. Variable Length Subnet Mask
VLSM
Block sizes
CIDR MASK HOSTS Block Size
/25 128 126 128
/26 192 62 64
/27 224 30 32
/28 240 14 16
/29 248 6 8
/30 252 2 4

11. VLSM Design