Using subnet masks: IP 10.15.1.1 with mask 255.255.0.0 will not be able to see IP 10.17.1.1 with mask 255.255.0.0, correct?
So why would I ever want to VLAN a switch unless if I needed to separate two servers plugged into the same switch with the same IP address?
I don't see the point of VLAN switches.
Best Answer
Because systems can modify their IP and/or subnet to see broadcast traffic on other networks on the same switch.
As a practical example: Imagine that in your scenario 10.17.0.0 is the high value network, and an attacker controls a system on the switch which currently has IP 10.15.1.96
The attacker sets up a network alias for 10.17.1.96 and proceeds to perform an ARP poisoning attack to MiTM all your traffic.
It would not have been possible for them to pivot to the 10.17.0.0 network if you had used vlans instead of a switch.