"Shared to other computers" question

[Ken Taylor <di604admin embarqmail com>]

Several months ago I setup a PC to share a VPN connection among the PCs 
on my LAN. A CentOS 7 box serves as a "gateway" and accesses the VPN 
using the openvpn client.

NIC1 connects to my router using DHCP on the router to obtain an IP 
address such as 192.168.0.116.

NIC2 is set to "Share to other computers". ifconfig shows this device to 
have the address 10.42.0.1.  By connecting another PC with a hard coded 
IP address in the 10.42.0.xxx range to a switch and thereby to the dual 
NIC machine, my second PC can connect to the Internet. MAGIC :-)

I then decided to do a little daisy chaining.  I added a second NIC to 
the second PC. I configured that interface to "Share to other 
computers." This connection gained the IP address 10.43.0.1 Cool.

I put the first dual NIC PC in "production" between my Internet 
connection and my LAN. I installed a DHCP server on the box and it 
serves up 10.42.0.xxx addresses to my LAN PCs.  Works great and has been 
in use for about 3 - 4 months.

Today I needed to setup something to do some firewall experimenting.  I 
plugged a test PC with 2 NICs to my LAN with NIC1. It received an IP 
address 10.42.0.xxx from my DHCP server. So far, so good.

I configured the second NIC as "Shared to other computers" as described 
above.  This time the second NIC received the address 10.42.0.1 which 
will not work. That is the address of the first dual NIC PC.

I have redone this several times. I also tried an Ubuntu 16.04 PC.  I 
still get the 10.42.0.1 address on the second NIC. I am at a loss.

Was the original 10.43 address a fluke?
Perhaps a newer version of network-manager-applet is hosed?
Something I need to configure manually in the firewall to cause an new 
subnet to be assigned to NIC2?  I really have no idea where the 10.42 
address came from in the first place.

The test PC is running CentOS 7.4 with network-manager-applet 1.8.0.3.


Any advice appreciated.

TIA,

Ken