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Scenario 4 — Additional address on ETH0 for a second subnet on the same switch#

Level: intermediate · Devices: EdgeBox, RevPi (eth0)

Additional address on ETH0

When this applies#

A device is physically on the same network as eth0 — same switch, no router in between — but uses an address from a different range. Typical examples:

  • a meter or inverter still on its factory default IP, such as 10.0.0.50;
  • an installer set the device up for their laptop and never changed it;
  • a separate OT range that shares the same switches as the IT range.

The device has no gateway that knows about the Octave device, so a route can't help. Instead, the Octave device joins that subnet too with an additional address on eth0.

Tip

Additional address or primary route? Ask: is there a router between the Octave device and the target? No router → additional address (this page). Router → primary route (Scenario 5).

What the device needs#

Field Value (example)
eth0 main address unchanged — DHCP or 192.168.0.225/24
Additional IPs / CIDRs 10.0.0.16/24 — a free address in the meter's subnet, with prefix

The additional address is attached as eth0:1, the next one as eth0:2, and so on. It never gets a gateway, and it's re-applied automatically after every DHCP event, so it works with DHCP and Static mode alike.

Configure#

  1. Find a free address in the target's subnet. Ask the site, or pick one far from the device's own address. Avoid .1 and .254, which routers often use.
  2. Open Edit configuration → Primary interface (eth0) – advanced.
  3. Tick Enable additional primary network addresses.
  4. Enter 10.0.0.16/24 under Additional IPs / CIDRs. Use ＋ Add address for more.
  5. Press Test, then Apply.

Additional addresses on eth0

Additional addresses on eth0

Check it#

  1. Troubleshooting → Network Overview: eth0 lists both addresses, and there's a route 10.0.0.0/24 on eth0 with no gateway.
  2. Connectivity to 10.0.0.50 with ARPing on eth0. An ARPing reply proves the meter is on the same cable.
  3. Port / Service Scan 10.0.0.50 with the Modbus preset.

Watch out for#

  • The address is already in use. Run Connectivity with ARPing to the address you want before adding it. No reply = free.
  • Overlap with the Octave range. An additional address in the eth1 range sends Octave devices without a direct link route to eth0. That's sometimes intended — see Scenario 6.
  • Prefer to fix the device. An additional address is a workaround. If the site agrees, re-addressing the meter into the main subnet is cleaner.