These defaults can be changed, see the Virtualbox Manual. If you start another guest set to NAT, that guest will also get 10.0.2.15 for its first NAT-connected card, 10.0.3.15 for the second, etc. If you add another network card to the guest and set its network to NAT too, that card will get 10.0.3.15, and so on. The default IP address given to the first NAT-connected network card in a guest is 10.0.2.15. Other guests, internet and the LAN can connect to the NAT-connected card via: host.ip.add.ress:portnumber The host can connect to the NAT-connected card via: localhost:portnumber. Other NAT-connected guest network cards can also talk to this particular network card only though forwarded ports. The host, LAN, and internet can only talk to that guest's network card through forwarded ports. NAT allows the guest's network card to talk to the host, the host's LAN, and the internet. Each NAT "router" only connects to one network card in one guest. NAT behaves like a house router with only one LAN port. Rather, "Sandbox" is a network setup using some of the above networking types and a router/firewall guest to isolate a guest or set of guests in a private "lab" that cannot see or access the host LAN, but can use the host's internet connection. "Sandbox" This is not an official Virtualbox network type and is not in the "Attached to:" dropdown. NAT network (called "Network Address Translation Service" in the manual)
Virtualbox provides these forms of networking in the "Attached to:" dropdown: Hopefully a picture might be worth a thousand words. This thread shows pictures of the more common Virtualbox networks. There is also a concise table in the manual, section 6.2, that shows the kinds of connections that can be set up and what communications can be had. The Virtualbox manual, section 6, has lots of good information on the types of Virtualbox networks that are provided with the pre-built Virtualbox installers.