VLAN flooding
A switch learns MAC addresses per port. Unknown unicast is flooded inside that VLAN only. Flooding is not routing. A router picks a next hop from an IP table and rewrites the layer-2 header when the packet leaves.
DHCP leases
DHCP hands out address, mask, gateway, and DNS for a limited lease. A green link light only means the cable layer is up. When the lease dies and renew fails, the host is off the network even if LEDs still blink.
RAID is not backup
RAID 1 mirrors. RAID 0 stripes with no safety. RAID 5 stripes with one parity disk and hurts during rebuild. None of that replaces a copy stored somewhere the array cannot delete.
Default gateway
The gateway is the first hop off the subnet. Wrong gateway looks like “the internet is down.” Check the host route to 0.0.0.0 before you blame the WAN circuit.
DNS vs path
Names that fail and addresses that work is DNS. Addresses that fail is path or filter. Test with an IP before you rebuild routing.
Cable vs config
A bad drop, a shutdown port, and a wrong VLAN all look like “no network.” Work layer 1, then 2, then 3. Do not start at the firewall.