In vSphere 5.x, it is possible to use static routes to allow migration with vMotion across subnets, although this is not officially supported. vSphere 6.0 introduces TCP/IPv4 and multiple netstack instances. vMotion uses TCP/IPv4 to acquire a netstack instance and default gateway to support vMotion across subnets, and this configuration must be used instead of static routing. If you upgrade an environment that was using static routing to vSphere 6.0, you cannot use migration with vMotion across subnets until you update the configuration to use TCP/IPv4.