Subnetting (Part 3 - VLSM)
Day 15
Subnetting Class A
The process of subnetting Class A, Class B, and Class C networks is exactly the same. For example, if we are required to subnet a 10.0.0.0/8 network creating 2000 subnets, we use the same formula. Borrowing 11 bits, we create 2048 subnets. The subnet mask is 255.255.224.0. The prefix length used is /19.
Now let's find the number of usable addresses. There are 13 bits remaining. Using the same formula we always used to find the number of usable host addresses, we can figure out that there are 8190 hosts per subnet.
VLSM
VLSM (Variable-Length Subnet Masks) is the process of creating subnets of different sizes, to make the use of network addresses more efficient. Until now, we learned FLSM (Fixed-Length Subnet Masks) - all subnets use the same prefix length. For example, to subnet this example network below, we have to use VLSM.
Using VLSM, we can assign different subnet sizes to each LAN. In VLSM, subnets are assigned in order, from the largest to the smallest. So, in our example above, we should first assign the subnet to Tokyo LAN A:
Network address:
192.168.1.0/25Broadcast address:
192.168.1.127/25Total number of usable addresses: 126
We divided our network into two and assigned the first half. Using VLSM, we can assign smaller subnets. The second largest LAN is Toronto LAN B:
Network address:
192.168.1.128/26Broadcast address:
192.168.1.191/26Total number of usable addresses: 62
The third is Toronto LAN A:
Network address:
192.168.1.192/27Broadcast address:
192.168.1.223/27Total number of usable addresses: 30
For Tokyo LAN B:
Network address:
192.168.1.224/28Broadcast address:
192.168.1.239/28Total number of usable addresses: 14
And finally, a point-to-point connection between two routers:
Network address:
192.168.1.240/30Broadcast address:
192.168.1.243/30Total number of usable addresses: 2
Not also we were able to provide addresses for all LANs but we even saved some address space thanks to VLSM.
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