This is part of "a setup to contribute to FreeBSD"
For this example we'll be using VirtualBox.
Things you need
ISO of the latest -CURRENT snapshot | you can download them from here
Setup VirtualBox
First, setup a Host-Only Networking Adapter, make sure to disable the DHCP Server.
Assuming the Host-Only Network now is vboxnet0 with the IP address 192.168.56.1/24, you will see vboxnet0 interface on your host.
Create a new virtual machine, in the network section, enable Adapter 2 and attach it to the Host-Only Adapter and set the name to vboxnet0. If you plan to do network programming, you might as well set Promiscuous Mode to Allow All.
Installing FreeBSD
Now you will need to install FreeBSD. Chapter 2. Installing FreeBSD section of the handbook has this covered. Make sure that you use ZFS, this will be handy later.
For the network setup, use em0 and set it to use DHCP.
After the installation is complete, you will need to configure em1 as well. Here are the commands to do so.
sysrc ifconfig_em1="inet 192.168.56.13 netmask 255.255.255.0
Shut down the machine by running shutdown -p now and remove the Disc from the VM.
Logging from the Host
If everything was done properly, now you can SSH from the Host. Since VirtualBox's VGA is not very easy to use, an SSH connection will allow you to copy and paste content from your terminal.
ssh -l myuser 192.168.56.13
Alternatively, if you want to log-in as root, edit the /etc/ssh/sshd_config file and change PermitRootLogin to yes, don't forget to uncomment and restart the daemon! 
What now?
Now go back to the Setup and let's configure what's needed for Src and Ports!
TODO
- Maybe setup instructions using Vagrant?
- Setup instructions with bhyve.
- Setup instructions using the official VM images from RE.