Proxmox VE 7.x Routed Subnet Configuration

interfaces

# Config of PVE networking with one virtual subnet (10.0.4.0/24), connected and routed to a physical subnet (10.0.0.0/24)
# This file should be saved at /etc/networks/interfaces
# run the following to make active: > ifreload -a 


auto lo
iface lo inet loopback

# this is the physical interface of the machine
###########

# switching it on automatically
auto enp0s31f6

# interface definition
iface enp0s31f6 inet static
         # the public IP on the port, in the "regular" subnet (here, connected to a physical router)
         address 10.0.0.11/24
         # IP of the physical router
         gateway 10.0.0.1
         # configuring IP forwarding to the virtual subnet
         post-up echo 1 > /proc/sys/net/ipv4/ip_forward
         post-up echo 1 > /proc/sys/net/ipv4/conf/enp0s31f6/proxy_arp
 
# the "virtual subnet"
################

# switching it to auto same as above
auto vmbr0
# interface config:
iface vmbr0 inet static
         # defines the "private" PVE subnet that is however routed out to the "regular" network. 
         # the *ROUTER* above (10.0.0.1) has a static route of 10.0.4.0/24 > 10.0.0.11 (which is then forwarded in PVE, the post-up... line)
         address 10.0.4.1/24
         bridge-ports none
         bridge-stp off
         bridge-fd 0
 
         # adding routes
         # this route leads back to the 10.0.0.0/24 "Regular" subnet on the physical port
         up ip route add 10.0.0.0/24 dev enp0s31f6
         
# VM guests can then simply have a 10.0.4.0/24 IP and use 10.0.4.1 as GW. This isn't using a VM instance to route or for firewalling or such. That may be another way to do it.