how to migrate a virtual machine on a single server esxi host to ovf

HI How do you use the ovftool to migrate a virtual machine (VMX) on a single server esxi 5.1 host to an ovf file? the syntax requires you to specify a data center . However I do not have a datacenter, because I only have a single esx host.

example: ovftool vi://username:pass@my_host/my_datacenter/vm/ my_vm_folder /ovfs/my_vm.ovf

What do I put for datacenter if I do not have one.

thanks!