Subnet calculator

Machine cells, robot lines, and process skids each deserve their own subnet: broadcast traffic stays local, addressing stays predictable, and the firewall between cell and plant network gets a clean rule ("this subnet may reach the historian, nothing else"). This tool sizes those subnets.

Picking a size

Count devices, double it for growth and spares, then round up to the next CIDR block: /29 (6 usable), /28 (14), /27 (30), /26 (62), /25 (126), /24 (254). A /24 per cell is the common habit — generous, readable (one octet varies), and cheap in private RFC 1918 space (10.x, 172.16–31.x, 192.168.x).

Conventions that prevent 2 a.m. confusion

Reserve .1 for the gateway, keep .2–.10 for infrastructure (switches, firewalls), assign PLCs and controllers low, I/O and drives in blocks by equipment number, and document every static address. DHCP reservations beat static configuration on devices that support them — one place to see the whole cell.

OT specifics

Never overlap cell subnets with the plant network or with another vendor's skid default (everything ships as 192.168.1.x — renumber on arrival). If two identical skids must talk, NAT at the cell boundary or renumber one. And keep multicast (EtherNet/IP implicit I/O, PROFINET RT) inside the cell with IGMP snooping so it never crosses upstream.

See also

• Choosing industrial network topology
• OT zones and conduits (IEC 62443)