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Network Interfaces Creating two network interfaces in Linux sudo ip link add eth0dummy1 type dummy sudo ip link add eth0dummy2 type dummy Setting those interfaces up sudo ip link set up eth0dummy1 sudo ip link set up eth0dummy2 Showing existing interfaces ip addr list MAC is assigned to eth0dummy1 and eth0dummy2 interfaces Bridge Creating the bridge sudo brctl addbr dummybridge Setting the bridge up sudo ip link set up dummybridge Adding interfaces to the bridge sudo brctl addif dummybridge eth0dummy1 eth0dummy2 Showing existing bridges sudo brctl show Showing virtual ports and MAC addresses sudo brctl showmacs dummybridge virtual port 1 attached to eth0dummy1 interface MAC and port 2 to eth0dummy2 MAC Router Converting the bridge to a router by assigning a L3 address. sudo ip addr add 172.16.0.1/24 dev dummybridge Reference

The routing table has the following entries by default with Virtual Box is used with NAT.  Destination  Gateway  Meaning  0.0.0.0       10.0.2.2  The traffic to any undefined network/IP address will be forwarded to the default gateway at 10.0.2.2     10.0.2.0       0.0.0.0  The traffic to the network 10.0.2.0/24 does not need any special gateway. The traffic can be directly forwarded.    169.254.0.0  0.0.0.0  The Link-Local address . If a DHCP server is not available, the server broadcast the it's own IP assignment using the link local address. See ARP.    172.17.0.0  0.0.0.0 The docker bridge since this machine has docker installed.

VirtualBox uses a NAT by default to allow each VM to reach internet. https://www.nakivo.com/blog/virtualbox-network-setting-guide/ Default IP assigned to the VM is 10.0.2.15 Default Gateway is 10.0.2.2 Read more: https://www.nakivo.com/blog/virtualbox-network-setting-guide/

Hub Layer: Physical Layer Connects network segments (wires) and broadcast traffic (without knowing to know MAC).  Bridge Layer - Data Link Layer Connects two network segments (wires) and forward packets based on the MAC.  Can be a software or a hardware.  Switch Layer - Data Link Layer Multiple bridges are implemented in a switch hence it can forward traffic between more than two network segments based on the MAC.