IP networking for automation engineers

Address and mask

An IPv4 address is 32 bits, written as four bytes: 192.168.10.25. The subnet mask tells which part is the network: with 255.255.255.0 (= /24, 24 network bits) all addresses 192.168.10.x are in the same subnet.

Two devices talk directly only if they are in the same subnet; otherwise they need a default gateway (router). A PLC configured 192.168.0.10/24 cannot reach 192.168.1.20/24 without a router, even on the same switch. Calculate with the IPv4 subnet calculator.

Private ranges (RFC 1918)

10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 — not routed on the Internet, used in plants. Avoid overlapping ranges between machines that will later be connected to the same plant network; agree an addressing plan early.

Practical rules

• One subnet per machine or line, with the PLC on a fixed address; document it on the cabinet.
• Do not leave devices on DHCP if a controller expects a fixed address.
• Duplicate IP addresses cause intermittent faults — check with the engineering tool's "accessible devices" or a network scan only where allowed.
• VLANs separate traffic on shared switches (e.g. control, video, office).
• Real-time Ethernet protocols (PROFINET RT, EtherCAT) use their own EtherTypes and do not route through IP routers.

Ports of industrial protocols (IANA)

Firewalls between cells must open exactly the ports in use — and nothing else.