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Go into the menu  File→Tools→Network Manager... in VirtualBox and create a new network called vboxnet1 for example, and enter IP 10.100.2.2 / 255.255.255.0 (no DHCP server). This will be used on NIC2 below. On Linux/MAC you need to allow this IP-range in by creating /etc/vbox/networks.conf and specifying allowed ranges there. For example, to allow 10.0.0.0/8 and 192.168.0.0/16 IPv4 ranges as well as 2001::/64 range put the following lines into /etc/vbox/networks.conf:

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