I managed to resolve this and thought I would share the config that worked for me, i think this is all based on stuff out there, but I could find anything with it all ...
So this is based on a hetzner EX host with an additional /29 subnet.
NOTE: fake ips below.
Main IP = 123.123.123.321
GW from Hetzner = 123.123.123.322
Subnet 123.123.321.123/29 (5 usuable IPs, 123.123.321.124-8)
Built the server using heztner installimage in rescue mode, went with debian 9.5, default config, installed proxmox as per the Heztner guide on their site, and then editing the networking config as per below
##### Configuration of host server Ethernet Adapter IP #######
auto enp0s31f6
<---- this is the name of my Ethernet adapter, yours may be different, check with "ip link" from console
iface enp0s31f6 inet static
address 123.123.123.321
<---- Main IP for server as per Hetzner welcome email
netmask 255.255.255.255
<----- different to welcome email subnet
gateway 95.216.7.129
<---- GW IP for server as per Hetzner welcome email
pointopoint 95.216.7.129
<---- GW IP for server as per Hetzner welcome email
##### Configuration of host server IP routes to allow for additonal subnet routing#######
auto vmbr0
iface vmbr0 inet static
address 123.123.123.321
<---- Main IP for server as per Hetzner welcome email
netmask 255.255.255.255
bridge_ports none
bridge_stp off
bridge_fd 0
bridge_maxwait 0
up ip route add 123.123.321.124/32 dev vmbr0
<---- note the /32 not the /29 from the welcome email
up ip route add 123.123.321.125/32 dev vmbr0
up ip route add 123.123.321.126/32 dev vmbr0
up ip route add 123.123.321.127/32 dev vmbr0
up ip route add 123.123.321.128/32 dev vmbr0
###### finished #######
Next is building your VMs, i have done a couple but the principle is the same.
NOTE: this will not work with a live / netinstal as internet access is not available till post build (possibly not true as there is a possibility to jump out of installers to shell to add routes, but for now this assumes you can install without a working connection).
Ubuntu config
##### Ethernet Configuration ######
network:
version: 2
renderer: networkd
ethernets:
ens18:
<---- this is the name of my Ethernet adapter in the VM, yours may be different, check with "ip link" from console
addresses: [123.123.321.124/32]
<----- IP from subnet allocation that has a routing option set on host server, mask must be /32
gateway4: 123.123.123.321
<---- Main IP for server as per Hetzner welcome email
routes:
- to: 0.0.0.0/0
via: 123.123.123.321
<---- Main IP for server as per Hetzner welcome email
on-link: true
nameservers:
addresses: [8.8.8.8,8.8.4.4]
###### finished #######
Debian config
##### Ethernet Configuration ######
# The loopback network interface
auto lo
iface lo inet loopback
# ethernet
auto ens18
<---- this is the name of my Ethernet adapter, yours may be different, check with "ip link" from console
iface ens18 inet static
address 123.123.321.124
<----- IP from subnet allocation that has a routing option set on host server
netmask 255.255.255.255
<---- must be this and not the mask from the welcome email
pointopoint 123.123.123.321
<---- Main IP for server as per Hetzner welcome email
gateway 123.123.123.321
<---- Main IP for server as per Hetzner welcome email
###### finished #######
I've also got this working with a Mikrotik Cloud Hosted Router, if anyone needs those CMDs to get the CHR going let me know.
I haven't figured IP6 yet ... TBH I dont know enough about it to try
Anyone fancy helping ?
I hope this helps someone out there !
