Running multiple dhcp client containers: port is already allocated

Host: Ubuntu Server 18.04, with an ISC-DHCP-Server
Container from Image: Ubuntu:18.04, image with a dhcp-client
Host and container are up and running OK.

When I try to launch a second dhcp-client-container, it fails with:
docker: Error response from daemon: driver failed programming external connectivity on endpoint Client2 (xxxxxx): Bind for failed: port is already allocated

I know that the same host port cannot be binded twice, but how can I then test a second client?
(DHCP uses UDP port 67 on server and UDP port 68 on clients)

Tips are welcome. Thanks.