Hardware in regular Computer amounts to One simple thing.
A collection of Chips that change and evaluate few states of Voltages usually relate to as 0 or 1. There are few basic types of core chip components, a variety of Gates, Flip Flops, Comparators, etc. The more sophisticated Chips are clever combos of the basic chips. Past the Core Chips every thing is Software.
However on the basic level the Software is embedded in chips (like PROMS), because it can not dynamically changed it is more stable then regular software, and being on chips (and thus not being depended on peripherals like Hard Drive) it is very fast.
Being on chip level it can not be build by individual (unless you have few millions to spare).
Cable/DSL Routers are actually small hardware computers with CPU, Memory, and embedded Software (I.e. Firmware).
The closet you can come to this it by building a computer using available Firewall software and embed the Software in PROM. There are commercial units that would allow you to interface through PROM rather then Hard Drive.
However it will be cheaper to by a Regular Cable/DSL Router.
:sun: