The router is the centerpiece of internetworking. It's the device that stitches networks together into internetworks and makes them useful. So, if you can learn how to manage routers properly, you can pretty much manage an internetwork. The network administrator's single point of control over router behavior is the configuration file, called the config file for short. The config file is one of only two permanent files on a router. The other is the IOS software, which is general in nature and cannot be altered by customers. The config file, then, is the network administrator's single point of control over the network. It's at the center of the router operations, with IOS referring to it hundreds of times per second in order to tell the router how to do its job.
Although the config file is the key tool, at first, it can seem hard to understand. This is because the config file is unlike the kinds of files most of us are used to. You can't put a cursor inside one and edit it in real time like you would, say, a word processor document. You can't compile it and debug it the way computer programmers turn source code into executable code. Config files are modified by entering IOS commands and then viewing the new configuration to see if you achieved the desired results.