Question

How does the logical process of combining bits known as ANDing work?

What is a Variable Length Subnet Mask (VLSM), and how is it created?

Answer 1

In VLSM, subnets use block size based on requirement so subnetting is required multiple times as VLSM is used for less waste IPs as in FLSM we have fixed size of all subnet even we need less IPs.

eg
Sales and Purchase: 120
Development: 50
Accounts: 26
Management: 5

As here if we use FLSM we have 128 size subnet of 4 in quantity as we have to select maximum size to satisfy all subnets where wastage are obvious high but to overcome this issue we have VLSM in which size varies as per the department requrement and subnet mask will be change as per the requirement where in FLSM it keep same for all 4.

FLSM
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Subnet Name	Needed Size	Allocated Size	Address	Mask	Dec Mask	Assignable Range	Broadcast
A	120	126	192.168.0.0	/25	255.255.255.128	192.168.0.1 - 192.168.0.126	192.168.0.127
B	120	126	192.168.0.128	/25	255.255.255.128	192.168.0.129 - 192.168.0.254	192.168.0.255
C	120	126	192.168.1.0	/25	255.255.255.128	192.168.1.1 - 192.168.1.126	192.168.1.127
D	120	126	192.168.1.128	/25	255.255.255.128	192.168.1.129 - 192.168.1.254	192.168.1.255

VLSM
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Major Network: 192.168.1.0/24
Available IP addresses in major network: 254
Number of IP addresses needed: 201
Available IP addresses in allocated subnets: 224

Subnet Name	Needed Size	Allocated Size	Address	Mask	Dec Mask	Assignable Range	Broadcast
A	120	126	192.168.1.0	/25	255.255.255.128	192.168.1.1 - 192.168.1.126	192.168.1.127
B	50	62	192.168.1.128	/26	255.255.255.192	192.168.1.129 - 192.168.1.190	192.168.1.191
C	26	30	192.168.1.192	/27	255.255.255.224	192.168.1.193 - 192.168.1.222	192.168.1.223
D	5	6	192.168.1.224	/29	255.255.255.248	192.168.1.225 - 192.168.1.230	192.168.1.231

As FLSM uses same size subnet we need more bits as in VLSM we can use wastage IPs as well

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