You're viewing the documentation for the experimental version of Vagrant (the 1.x series). The current stable series is 1.0.x and the documentation can be found here. You can learn more about the Vagrant versioning scheme here.
Configuration key: config.vm.auto_port_range
Default value: 2200..2250
Forwarded ports defined for your VM can sometimes collide with other VMs. For example, you might forward port 80 to 8080 in multiple virtual machines. Vagrant has the ability to automatically resolve forwarded port collisions for you when they’re detected. When these are resolved, it chooses a port from this configured range. The value of this configuration parameter should be a Ruby Range object.