Process for configuring network settings on a headless rack mount device
Solution 1:
There are multiple approaches to this problem. Let me at least point you in the right direction.
Most SOHO routers come with a default IP address in the RFC 1918 address space. This configuration may be modified by connecting a host usually by ethernet cable and then pointing a browser to that default address. Sometimes the devices offer DHCP so that the admin client doesn't have to manually set an IP in the correct network to connect for the first time.
Another common approach is to connect out-of-band, usually over serial cable. Higher end routers and switches come with this cable and end users install the device, power it on, and connect to the serial port from a local machine. From there, they configure the IP to match their network.
One final approach would be to set the ethernet port on the device to request a DHCP address. You would then plug the device into a DHCP network such as a small office network or home network. You could use process of elimination to determine what the new device's IP address is and then connect to a web UI or similar interface from there.
Solution 2:
Initial setup of such devices is often done first through a serial console. Another option is a static IP address on a common subnet. That forces a technician to either put the device on a staging subnet or directly connect to the device using the same subnet statically assigned to a laptop or other computer. A final option is acquiring a DHCP address, which is pretty lame and only reserved for home networking equipment.
Solution 3:
The answer to this depends on what kind of role the device plays, its location in the network and the quantity of devices that need to be managed. A very small number of devices could do something like the default address approach (i.e. SOHO devices). If typical deployment calls for less than 50 or so boxes and there's a centralized location then some sort of basic serial console setup is fine. This approach can (and does) scale to large numbers of devices in the case of network hardware, but this is in some part a function of history.
If the intention is for many hundreds (or thousands) of devices deployed on a wide basis then working out some kind of mechanism for dynamically pushing configurations, firmware, etc can be a big win. Using DHCP to pull not only an address but also some measure of custom configuration (link to central config server, pull down image based on serial/MAC, etc) has been an approach that has been used to good effect by some vendors.