For individual containers —

For a container named c with a virtual interface ve-c,

1. Create a systemD network configuration file in your host machine ...

; /etc/systemd/network/N-ve-c.network [Match] Name=ve-c Driver=veth [Network] ; Assign an address and subnet to this interface; the IP used will be the gateway for containers Address=169.254.0.1/24 ; Turn this on to make IP packets originating in the container be "masked" with the IP of the host; this allows the container to reach the public internet through the host's other interfaces ; IPMasquerade=yes 

... assigning it an IP and a subnet, and instructing it to operate a DHCPServer on that interface

2. Create a systemD network configuration file on the container ...

; /etc/systemd/network/1-host0.network [Match] Virtualization=container Name=host0 [Network] ; Address this container will have, must be in the subnet assigned to this container's interface on the host Address=169.254.0.2/24 ; Address assigned to this container's interface on the host Gateway=169.254.0.1 ; The DNS server to use; only needed if IPMasquerade is on ; DNS=1.1.1.1 

... assigning it an IP and the subnet assigned on the host-side, and informing it of what the IP of its gateway is.

3. Ensure that systemd-networkd is the only network manager running on the host and the container: systemctl start --enable systemd-networkd.service

4. Restart systemd-networkd on your host machine: systemctl restart systemd-networkd

5. Start the container: machinectl start c

6. The container should have had a single static IP address assignd to it: machinectl

MACHINE CLASS SERVICE OS VERSION ADDRESSES c container systemd-nspawn arch - 169.254.0.2… 1 machine listed. 

To network between containers —

If you want your containers to be able to talk to each other as well, bridge their interfaces

For containers named c1, c2, c3, ... with virtual interfaces ve-c1, ve-c2, ve-c3, ...

1. Create a bridge device on your host machine:

; /etc/systemd/network/N-br-container-group.netdev [NetDev] Name=br-container-group Kind=bridge 

2. Instruct systemd-nspawn to bridge the virtual interfaces created for each machine to the specified network bridge:

; /etc/systemd/nspawn/c{1,2,3,...}.nspawn [Network] Bridge=br-container-group 

3. Continue as before, assigning an IP and subnet to the interface:

; /etc/systemd/network/N-br-container-group.network [Match] Name=br-container-group ; We are no longer using a veth driver, so ask systemd-networkd to match a bridge device instead ; Driver=veth Kind=bridge ... 

Note: you do not need to create a systemD network configuration file for the virtual interfaces of the containers like before; the required configuration will have been performed by systemd-nspawn on encountering the Bridge=br-container-group instruction.

4. And instructing each container on what IP and gateway to use, as before; the containers are not aware that they are bridged, so the container-side configuration is identical to the previous case.

Note: Each container shares the same gateway

Note: Each container has a different static IP address, each of which must nevertheless come from the same subnet assigned to the bridge interface

5. Ensure systemd-networkd is running, and start the container as before

6. The containers can now communicate with each other, you can confirm this by running a lightweight HTTP server on each container and sending requests between them

The following worked for me:

Assign static IP address to systemd-nspawn container (private networking mode).

Note: I just modified the DHCPServer=no option because DHCP is no needed in the static IP assignment and to prevent the conflict with the dnsmasq DHCP server service running on the host in my use case.