Without segmentation, every device on your network shares the same broadcast domain. When one host sends an ARP request, every other host must process it. Put devices on a flat network and you'll see constant broadcast traffic eating bandwidth. Worse, any compromised device can sniff traffic from all others. Segmentation fixes both problems by creating smaller, isolated broadcast domains.
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$ curl repovive.com/roadmaps/network-design/switching-and-vlans/why-segment-your-network
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