Infinite thanks to [ingo](https://raspberrypi.stackexchange.com/users/79866/ingo) for sharing these setup instructions online!

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[quick-setup](https://raspberrypi.stackexchange.com/a/108593)


# deinstall classic networking

sudo -Es   # if not already done
apt install libnss-resolve
apt --autoremove purge ifupdown dhcpcd5 isc-dhcp-client isc-dhcp-common rsyslog
apt-mark hold ifupdown dhcpcd5 isc-dhcp-client isc-dhcp-common rsyslog raspberrypi-net-mods openresolv
rm -r /etc/network /etc/dhcp


# setup/enable systemd-resolved and systemd-networkd

apt --autoremove purge avahi-daemon
apt-mark hold avahi-daemon libnss-mdns
ln -sf /run/systemd/resolve/stub-resolv.conf /etc/resolv.conf
systemctl enable systemd-networkd.service systemd-resolved.service


# create interface file for a wired connection

sudo -Es
cat > /etc/systemd/network/04-wired.network <<EOF
[Match]
Name=e*

[Network]
## Uncomment only one option block
# Option: using a DHCP server and multicast DNS
LLMNR=no
LinkLocalAddressing=no
MulticastDNS=yes
DHCP=ipv4

# Option: using link-local ip addresses and multicast DNS
#LLMNR=no
#LinkLocalAddressing=yes
#MulticastDNS=yes

# Option: using static ip address and multicast DNS
# (example, use your settings)
#Address=192.168.50.60/24
#Gateway=192.168.50.1
#DNS=84.200.69.80 1.1.1.1
#MulticastDNS=yes
EOF

reboot


[switch between wifi client and access point without reboot](https://raspberrypi.stackexchange.com/questions/93311/switch-between-wifi-client-and-access-point-without-reboot)

# setup wpa_supplicant as wifi client with wlan0

cat > /etc/wpa_supplicant/wpa_supplicant-wlan0.conf <<EOF
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1

network={
    ssid="TestNet"
    psk="verySecretPwassword"
}
EOF

chmod 600 /etc/wpa_supplicant/wpa_supplicant-wlan0.conf
systemctl disable wpa_supplicant.service
systemctl enable wpa_supplicant@wlan0.service


# setup wpa_supplicant as access point with ap0

cat > /etc/wpa_supplicant/wpa_supplicant-ap0.conf <<EOF
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1

network={
    ssid="RPiNet"
    mode=2
    key_mgmt=WPA-PSK
    proto=RSN WPA
    psk="anotherPassword"
    frequency=2412
}
EOF

chmod 600 /etc/wpa_supplicant/wpa_supplicant-ap0.conf


# configure interfaces

cat > /etc/systemd/network/08-wlan0.network <<EOF
[Match]
Name=wlan0
[Network]
DHCP=yes
EOF

cat > /etc/systemd/network/12-ap0.network <<EOF
[Match]
Name=ap0
[Network]
Address=192.168.4.1/24
DHCPServer=yes
[DHCPServer]
DNS=84.200.69.80 1.1.1.1
EOF


# modify service for access point to use ap0

systemctl disable wpa_supplicant@ap0.service
systemctl edit --full wpa_supplicant@ap0.service

# modify/insert only these lines: Requires=, After=, Conflicts=, ExecStartPre= and ExecStopPost= as shown:

[Unit]
Description=WPA supplicant daemon (interface-specific version)
Requires=sys-subsystem-net-devices-wlan0.device
After=sys-subsystem-net-devices-wlan0.device
Conflicts=wpa_supplicant@wlan0.service
Before=network.target
Wants=network.target

# NetworkManager users will probably want the dbus version instead.

[Service]
Type=simple
ExecStartPre=/sbin/iw dev wlan0 interface add ap0 type __ap
ExecStart=/sbin/wpa_supplicant -c/etc/wpa_supplicant/wpa_supplicant-%I.conf -Dnl80211,wext -i%I
ExecStopPost=/sbin/iw dev ap0 del

[Install]
Alias=multi-user.target.wants/wpa_supplicant@%i.service

# set wlan0 to run as client on startup

sudo systemctl enable wpa_supplicant@wlan0.service
sudo systemctl disable wpa_supplicant@ap0.service
reboot


# switch the service when desired (no stopping of services is required)

sudo systemctl start wpa_supplicant@ap0.service
sudo systemctl start wpa_supplicant@wlan0.service