When a VM moves from one subnet to another,
its IP address must change, which complicates
routing. It is well known that IP addresses
are both locators and system identifiers, so when
a system moves, its L3 identifier changes. In
spite of all the developments of mobile IP, it is
significantly simpler to move systems within one
subnet (within one L2 domain) than between
subnets. This is because the IEEE 802 addresses
used in L2 networks (both Ethernet and WiFi)
are system identifiers (not locators) and do not change when a system moves. Therefore, when a
network connection spans multiple L2 networks
via L3 routers, it is often desirable to create a
virtual L2 network that spans the entire network.
In a loose sense, several IP networks together
appear as one Ethernet network.