A maneira mais simples é definir a máquina virtual para usar a rede em ponte, ele irá obter um endereço IP em sua rede; este endereço IP deve ser acessível a partir de sua máquina host.
De Capítulo 6 do manual do VirtualBox :
With bridged networking, VirtualBox uses a device driver on your host system that filters data from your physical network adapter. This driver is therefore called a "net filter" driver. This allows VirtualBox to intercept data from the physical network and inject data into it, effectively creating a new network interface in software. When a guest is using such a new software interface, it looks to the host system as though the guest were physically connected to the interface using a network cable: the host can send data to the guest through that interface and receive data from it. This means that you can set up routing or bridging between the guest and the rest of your network.
Também é possível usar o modo de rede NAT , mas será necessário mexer com encaminhamentos de porta (não vale a pena, na minha opinião).