"Shared to other computers" connection

[Ken Taylor <di604admin embarqmail com>]

I am running Ubuntu 16.04 Mate on a Raspberry Pi. The intent is to build 
a router, firewall, vpn sharing box to replace a CentOS 7 machine which 
is performing the same functions. The built in NIC will connect to the 
DSL router (bridged mode) and a USB to Ethernet dongle will provide the 
LAN side connection.

The NetworkManager applet allows me to configure IPV4 on the second 
connection to "Shared to other computers."  This assigns an address of 
10.42.0.1 to the interface and allows me to connect other computers via 
a dumb switch and the computers pickup DHCP from the Pi and the Pi 
routes traffic to/from the Internet. Easy and Fantastic!

However... To test this I wish to connect my baby network to my existing 
LAN. It is served by a similar configuration on the CentOS machine. 
Sharing the second interface on this box also gets the default 10.42.0.1 
address so things get messed up if I daisy chain the baby/test LAN to 
the main LAN.

I should be able to configure dnsmasq to do several things which I need 
to do such as select a different sub-net. However, dnsmasq is invoked by 
network-manager and the normal dnsmasq configuration files do not have 
the desired effect. Long story short, here are the things I need help to do:

1 - set the shared subnet on the Pi to say 10.42.1.xxx

2 - set the range of addresses served by DHCP

3 - reserve certain IP addresses for certain devices by MAC

Any guidance would be appreciated.

TIA,

Ken