Will actually in theory on switched networks each portion
of a LAN is independent. This means that for instance that network
192.168.1.0/24 and 192.168.2.0/24 are using different switch ports that
communicate through a router, and also that are not sharing the same
physical network. Unfortunately sometimes people violate this principle by
putting on the same physical port multiple networks.
The reasons are manyfold:
·
You want to run a VM on
your host that can (silently) communicate with other devices and thus you want
to use a different network address plan.
·
You use devices that have
an embedded switch (e.g. Apple Airport Time Capsule NAS device) to
which you connect both your PC (with a publicly accessible IP address) and the
backup device that is not supposed to be accessed from the Internet and thus
living on a different network.
·
Some of your colleagues are
trying to hide some devices and thus are decided to use a network other than
the one used on the LAN.
·
You migrated your network
to a different addressing scheme but you forgot to update some devices that are
still configured with the old network.
·
Somebody attached (without
configuring it) a new device just purchased that is then using a different
network address.
So in essence there are
many reasons ranging from misconfiguration, to malicious users who attach
devices to the network hoping not to be discovered. Fortunately moderns devices
are rather verbose and advertise their presence for instance through MDNS (Multicast
DNS), IPv6 advertisements, and for sure ARP on IPv4
networks.
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