Both the Linaro OpenEmbedded and Android software stacks have ethernet support enabled by default using DHCP. This means your network must support DHCP for this to work.
Juno actually includes three physical ethernet ports; one on the front and two on the rear. See the image below extracted from the Juno Getting Started Guide:
On Juno r0 boards only the front ethernet port is functional.
On Juno r1 and later both the front 10/100 and rear gigabit ports are functional, though their enumeration will depend on your kernel variant and deliverables release:
Where `--' corresponds to the ethernet port being unavailable for that configuration.
The port labelled "config ethernet" on the rear of the board is for Arm internal use only.