Why a laptop cannot reach an IED — and how to read the result
Every IPv4 device decides on its own whether a destination is on its own LAN. It applies its own subnet mask to its own address and to the destination: if both give the same network address, it sends the frame directly (after an ARP request for the destination's MAC address). If not, it sends the frame to its default gateway. If it has no gateway, it cannot send at all.
Because each side uses its own mask, two devices can disagree. A laptop on 10.13.0.200/24 sees an IED at 10.13.0.14 as local and sends directly. The IED on 10.13.0.14/25 sees 10.13.0.200 as outside its range 10.13.0.0–127 and sends its reply to its gateway. The request arrives, the reply goes to the router and is normally lost. Ping fails, but the IED's counters show that requests came in — a typical one-way fault on site.
The assistant repeats the decision from both sides and gives one of four results: can talk directly (same subnet on both sides), only through a router (different subnets, both have gateways), mask mismatch (the two sides disagree), or cannot talk. It also flags addresses that can never work: duplicates, the network or broadcast address of a subnet, 169.254.x.x link-local addresses (the adapter was set to DHCP and got no answer), and gateways outside the device's own subnet.
When the addressing is right, the next step is the service. IEC 61850 MMS uses TCP 102, IEC 60870-5-104 uses TCP 2404, DNP3 uses TCP 20000 and Modbus TCP uses 502. If ping works but the engineering tool does not connect, the cause is usually the Windows firewall, a switch port in a different VLAN, the service disabled on the IED, or the IED's limit on simultaneous client connections.
Formulas
- Network address:
- network = IP AND mask (bit by bit). Two addresses are on the same subnet when IP₁ AND mask = IP₂ AND mask — evaluated with each device's OWN mask.
- Usable range:
- first host = network + 1, last host = broadcast − 1, broadcast = network OR (NOT mask); usable hosts = 2^(32 − prefix) − 2 (for prefixes up to /30).
Common masks on substation networks
| Prefix | Mask | Addresses | Usable hosts |
|---|---|---|---|
| /24 | 255.255.255.0 | 256 | 254 |
| /25 | 255.255.255.128 | 128 | 126 |
| /26 | 255.255.255.192 | 64 | 62 |
| /27 | 255.255.255.224 | 32 | 30 |
| /28 | 255.255.255.240 | 16 | 14 |
| /29 | 255.255.255.248 | 8 | 6 |
| /30 | 255.255.255.252 | 4 | 2 |
Frequently asked questions
Do I need a default gateway on my laptop to talk to the IED?
Not if the laptop is in the IED's subnet — the traffic goes directly over the LAN. A gateway is needed only to reach other subnets. A gateway outside the laptop's own subnet is ignored by Windows.
Ping works but the IEC 61850 client cannot connect. Why?
The network is fine; check TCP 102. Common causes are the Windows firewall blocking the tool, the MMS server disabled or limited to a few clients on the IED, or an access list on a router or firewall in between.
Why does my laptop show 169.254.x.x?
The adapter is set to obtain an address automatically and found no DHCP server, so Windows picked a link-local address. Set a static address in the IED's subnet.
Can two devices with different masks talk?
Yes, if each sees the other inside its own subnet (for example /24 and /25 with both addresses in the lower half). It is still poor practice: all devices on one LAN should use the same mask.