DHCP Troubleshooting
When you log on to the Internet, the ISP uses DHCP (Dynamic Host Configuration Protocol) to assign temporarily an IP address to your terminal for the duration of the surfing session. In other words, your ISP leases an IP address to you for a period of time. When the lease expires, the client renews the old lease or gets a new IP address. You'll probably never receive the same IP address twice.

This happens transparently and one neither needs to know or care what that temporary IP address actually is. You transparently access websites and email with the help of the DNS (Domain Name Server) system. Once a new computer is set to accept DHCP, it will function happily across the office network.

When it comes to automation networks DNS names are seldom used for device identification. It would be impractical anyway because of the actions that must be taken in replacing a failed device .In the factory, the client (perhaps a PLC)needs to locate the server(s) - which are I/O devices - rather than the server locating the clients. More about this later.

If a printer fails in your office and you replace it with another, the computers must be manually rebooted or configured to recognize the new printer. Not a big deal in the office environment but an unacceptable situation on the factory floor? Why? Because one can't re-configure a PLC program just because a photo eye fails.

Constantly changing IP addresses are simply incompatible with the standard form of automation system where name tags are not generally used for device identification. When you replace a device, you need to know what IP address to set up in the replacement part. In fieldbus systems, network addresses are often set by dip switches, and some customers prefer or even dictate dip switches so that maintenance people can easily make replacements without PCs or external tools.