Bridging

Assume you have eth0 now and want to create bridge br0.

Using /etc/network/interfaces

Create /etc/network/interfaces.d/br0, fill in:

auto br0
allow-hotplug br0
iface br0 inet dhcp
    bridge_ports eth0
    bridge_waitport 60
iface br0 inet6 auto

Then ifup br0.

If it fails to create the bridge, apt install bridge-utils.

Using systemd-networkd

Create 3 files under /etc/systemd/network:

br0.netdev

[NetDev]
Name=br0
Kind=bridge
br0.network

br0.network

[Match]
Name=br0

[Network]
DHCP=ipv4
eth0.network

eth0.network

[Match]
Name=eth0

[Network]
Bridge=br0

Then systemctl reload systemd-networkd.