Google Cloud Compute – Setting /20 Subnet Mask for Internal Interface

Google cloud uses DHCP for assigning IP's to an instance. For some reason, they assign the address with a /32 netmask even though you're on your own /20 network. I have found that if I set the instances public IP to static I can then go into /etc/syconfig/network-scripts/ifcfg-eth0, change BOOTPROTO from DHCP to STATIC then manually set the IP settings and use a /20 or /24 subnet and it will survive reboots. However, after doing this, I lose the ability to communicate with that host on the internal network. If the instance is using DHCP parameters, I can communicate between hosts on the LAN without issue.

After reading around online, I found this article https://cloud.google.com/compute/docs/networking has a section talking about making changes to DNS and resolv.conf and to use the dhcp.lease config to do so. When I look in this file, I see that it has the 'option subnet-mask 255.255.255.255;' setting. If I change the netmask and restart the network, the changes are reverted.

Just for reference:

instance-2 is using default DHCP and has the IP 10.128.0.5
instance-4 is using my custom static config and has the IP 10.128.0.6

I have also compared the route table between an instance with a default DHCP address and an instance with my static IP settings.

instance-2 (DHCP):

[root@instance-2 network-scripts]# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         10.128.0.1      0.0.0.0         UG    100    0        0 eth0
10.128.0.1      0.0.0.0         255.255.255.255 UH    100    0        0 eth0
10.128.0.5      0.0.0.0         255.255.255.255 UH    100    0        0 eth0
169.254.169.254 10.128.0.1      255.255.255.255 UGH   100    0        0 eth0

instance-4 (Custom Static):

[root@instance-4 NetworkManager]# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         10.128.0.1      0.0.0.0         UG    100    0        0 eth0
10.128.0.0      0.0.0.0         255.255.240.0   U     100    0        0 eth0

I then manually added the different routes to instance-4:

[root@instance-4 NetworkManager]# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         10.128.0.1      0.0.0.0         UG    100    0        0 eth0
10.128.0.0      0.0.0.0         255.255.240.0   U     100    0        0 eth0
10.128.0.1      0.0.0.0         255.255.255.255 UH    0      0        0 eth0
10.128.0.6      0.0.0.0         255.255.255.255 UH    0      0        0 eth0
169.254.169.254 10.128.0.1      255.255.255.255 UGH   0      0        0 eth0

But that didn't resolve the issue either.

instance-4 network script:

TYPE=Ethernet
BOOTPROTO=static
DEFROUTE=yes
IPV4_FAILURE_FATAL=no
NAME=eth0
UUID=cde7258f-6857-4015-86de-6bb520fcd550
ONBOOT=yes
PEERDNS=yes
PEERROUTES=yes
MTU=1460
PERSISTENT_DHCLIENT="y"
NETMASK=255.255.240.0
IPADDR=10.128.0.6
DNS1=169.254.169.254
GATEWAY=10.128.0.1

instance-2 network script

TYPE=Ethernet
BOOTPROTO=dhcp
DEFROUTE=yes
IPV4_FAILURE_FATAL=no
NAME=eth0
UUID=cde7258f-6857-4015-86de-6bb520fcd550
ONBOOT=yes
PEERDNS=yes
PEERROUTES=yes
MTU=1460
PERSISTENT_DHCLIENT="y"

How can I properly get the interface to use a netmask other than /32 and still be able to communicate with other instances on the LAN?

The OS is CentOS 7

I'm needing a netmask other than /32 so that I can install FreeIPA. It will not install if the netmask is /32.