iocage handles network configuration for both shared IP and VNET jails transparently.
Configuring a shared IPv4 jail:
iocage set ip4_addr="em0|192.168.0.10/24" UUID
This will add an IP alias 192.168.0.10/24 to interface em0 for the shared IP jail at start time.
A better approach using VNET:
iocage set ip4_addr="vnet0|192.168.0.10/24" UUID
iocage set defaultrouter=192.168.0.254 UUID
For VNET jails a default route has to be specified, just like for a normal physical host.
Hints
To start a jail with no IPv4 address whatsoever:
iocage set ip4_addr=none UUID
iocage set defaultrouter=none UUID