But not necessarily a physical one. My guess is you are using Virtualbox NAT, instead of bridging.
If that's true, then your guest OS is connected to a virtual router provided by Virtualbox, as per:
"A virtual machine with NAT enabled acts much like a real computer that connects to the Internet through a router. The "router", in this case, is the VirtualBox networking engine, which maps traffic from and to the virtual machine transparently. The disadvantage of NAT mode is that, much like a private network behind a router, the virtual machine is invisible and unreachable from the outside internet; you cannot run a server this way unless you set up port forwarding (described below)."
Extracted from Chapter*6.*Virtual networking
And it's that virtual router who is actually reporting ftp as open to the outer world. If I'm right, then either change your vbox settings to bridged networking or carefully check NAT port forwarding. If I'm not, then try harder