Configure connection sharing with specific IP Address
In versions before 1.4.2, 10.42.0.x is hard-coded into NetworkManager. Forget about GUI-based solution or wait until the bug/feature request is fixed. Or fix it yourself for all of us. Or at least vote the bug on launchpad for faster resolution.
Either upgrade to Ubuntu 17.04, with version 1.4.4, or use the following command from Thomas Haller to set the host IP and class.
nmcli connection modify $CONNECTION_ID +ipv4.addresses 192.168.5.1/24
where $CONNECTION_ID if found via nmcli connection show. Afterwards, verify with nmcli connection show $CONNECTION_ID.