Some network newbies may have read books about networking and still doubt about what is the point of VLAN and Subnet. Why we need both ? ... It looks similar, it segment network into separate network (L2,L3) for security, smaller broadcast domain, etc.
So ... what is the key point of VLAN and Subnet ?
VLAN - is used for segment/separate L2 network. Key point of creating VLAN is for more security, lower broadcast domain, save costs. If computer shared same physical/equipment it usually on same L2 network, if you want to separate L2 network you need 2 sets of equipments. But if equipment/computer supported VLAN, we can put each computer/equipment to different L2 network while sharing same physical infrastucture (switches, wire, router, etc.).
Subnet - is used for address assignment. Key point of subnetting is to assign L3 network address (subnet) appropriate for usage (also sometimes for network address conservation). In typical setups, different VLAN also need different subnet. So ... subnetting usually done along with VLAN segmentation. Security and smaller broadcast domain is not main point of subnetting because it already done by VLAN instead.
You can also do have few subnets in one L2 network (VLAN) but ... anyway no security between two L3 network at all. All node in each L3 network still receive other L3 network packets broadcasts in same L2 network.
...
In short
VLAN = segment/separate L2 network
Subnetting = Assign appropriate L3 network address (subnet) to L2 network
FYI!
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