A DHCP address is recorded in the server's lease table and in a query from the panel's programming software, and the info screen shows it too. The address changes only when a lease lapses: a renewal at half the lease keeps the same address. A DHCP reservation on the panel's MAC ends the guessing.
When a panel takes its address from DHCP, that address is written down in three places: the DHCP server's lease table, a network query run from the panel's programming software, and the info screen in the panel's own setup menu. The first two need no menu at the panel, so start there, and match each lease to the panel by the MAC address on its label. The address should hold steady too: RFC 2131 has a DHCP client renew at half the lease time, and a renewal keeps the same address. When a panel's address really does move, a lease lapsed somewhere, and the arithmetic below shows when.
Three places hold the address
| Where you look | What it shows | Reach for it when |
|---|---|---|
| DHCP server lease table | client address, name, MAC and lease expiry, per scope | you run the server, or the administrator can check it in a minute |
| Programming software network query | the address of every panel that answers on the subnet | someone else runs DHCP and you have the programming laptop |
| Info screen, in the setup menu | the current address on the panel itself | the first two return nothing, or several entries look alike |
Work the table top to bottom. The lease table changes nothing on the panel and takes a minute. The network query needs only the programming laptop. The info screen is the one check that needs hands on the panel, so it comes last.
What you need
- A PC on the panel's subnet, with the panel's programming software installed.
- The MAC address from the label on the panel or from the project sheet.
- Access to the DHCP server console, or the network administrator on call.
- An FTP client, for the check at the end.
Find the address in order
- Read the lease table. Open the DHCP server console, pick the scope the panel is wired into, and open its address leases. Match on MAC address, because the client names embedded devices report are rarely useful. Most server consoles also let you search the leases by a complete IPv4 address with a date range, which is how you chase an address you half remember. One matching lease hands you the address and its expiry; write both down.
- Query the network. Run the network query from the programming software, on the PC that sits on the panel's subnet. It reports an address for each panel that answers. Several answer: match on the panel or station name the software prints. None answers: the PC is on another VLAN, or the panel has no lease at all.
- Read the info screen. In the setup menu, the network or system info screen prints the current address, and on most panels the subnet mask and gateway beside it. If the touchscreen is dead, getting into that menu is its own job; password entry on an HMI panel with no touch screen covers the inputs you can still use.
Why the address moves at all
RFC 2131 sets the rhythm. A client renews at T1, half the lease time by default (the server's options can move that): a 24 h lease renews at 12 h. The renewal goes to the same server that issued the lease and keeps the same address. If the panel misses that window because it is powered off, it asks again at T2, 87.5% of the lease (21 h on that 24 h lease), this time to any server on the network. If both windows pass with the panel dark, the lease lapses, and the next boot starts the request over. At that point the server is free to hand out a different address, and on a busy scope it often does.
That gives a planning rule you can hold to: a panel keeps its address through any outage shorter than half the lease. Keep the lease at least twice the longest time the panel sits powered off. An 8 h night needs a 16 h lease; the common 24 h lease covers it with room to spare.
Example: the FTP shortcut that worked on Monday
Scenario: the panel logs alarm history to a USB stick and you pull the files by FTP. On Monday the address 192.168.1.57 worked. On Tuesday it refuses. The panel powers up with the line at 06:00 and down at 22:00, and someone has shortened the scope's lease to 1 h.
Arithmetic on the 1 h lease: renewals run every 0.5 x 1 h = 30 min, so the last one before shutdown lands at 21:30 and the lease dies at 22:30. The panel is off 8 h, so it wakes 7.5 h after the lease lapsed. A lapsed lease means the panel starts its request over, and Tuesday morning 192.168.1.57 answers for another device.
Same schedule on the old 24 h lease: the panel renews at 06:00 + 12 h = 18:00, before the shutdown, and the renewed lease would not expire until 18:00 the next day. Tuesday's 06:00 boot finds 12 h of lease still valid, the panel keeps 192.168.1.57, and nothing moves.
The lease length is the whole difference: 1 h lapses in the first hour the panel is dark, 24 h rides out the night. The fix below removes the dependence on lease length altogether.
How to check it worked
Ping the address first; on the same subnet expect replies within a few milliseconds. Then run the FTP session: connect to that address on port 21, the standard FTP control port, log in, list the alarm directory, and download one file, checking its size against what the panel reports. The session itself, from login to retrieval, is laid out in connecting an FTP client to an HMI panel over Ethernet. Pull the USB stick only with the panel powered down or the log paused; pulling it mid-write is how alarm files end up truncated. If ping answers but FTP refuses while the panel's web pages are also dead, that pairing has its own writeup: why the web server stops while FTP still works.
Stop the guessing for good
Recommendation: ask the network administrator for a DHCP reservation that binds the panel's MAC address to one address inside the scope, and do it for every panel you reach by address, FTP included. The server then hands that panel the same address at every lease, however short the lease is or how often the line powers down. If you run the scope yourself, a static address outside the assignment pool does the same job on the panel side.
When you add a notice board on the same network, give it the same five minutes of paperwork: one reservation per device, matched to the MAC on its label. The displays we build for shop floors live on those same switches and deserve the same treatment.
Questions
The lease table shows no lease for the panel at all. Now what?
Check the panel's link lights and confirm the PC you query from sits on the same subnet and VLAN. Then look at the scope on the server: a pool with no addresses left hands out nothing new. A panel with no lease prints 0.0.0.0 or an address starting 169.254 on its info screen, and neither reaches across the plant.
How long should the lease be on a plant network?
Server DHCP software for wired scopes commonly defaults to 8 days, which suits fixed machinery: the panel renews every 4 days and one address rides out a weekend shutdown. A 1 h lease is a deliberate change, typical of guest networks, and it moves machinery addresses whenever power drops. A reservation makes lease length irrelevant for a panel you reach by address.
Can I find the panel without any access to the DHCP server?
Yes, from the programming laptop. Ping each address on the panel's subnet, then read the ARP cache with the arp command: it lists every address that answered and the MAC that owns it. Match that MAC against the label on the panel. Slower than a lease table, and it needs no server login.
Should I use a reservation or type a static address into the panel?
A reservation keeps the panel in DHCP: the server binds the panel's MAC to one address in the scope, nothing changes on the panel, and the lease table keeps logging it. A static address typed into the panel must sit outside the pool the server assigns from, or it will eventually collide with a lease handed to some other device.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.