Stepping down 48V (from power over Ethernet) to 5V
As you correctly noted, a step down converter is the right thing to use.

You'll notice that all bigger semiconductor manufacturers have tools on their website to help you design a step-down converter from e.g. 48 V to 5 V for the current your device needs (500mA, as you say).

However, if your PoE-injector is truly standards-compliant (far from all are), then there will be no 48V on the Ethernet port until the device signals it needs that (wikipedia)


  The PSE can implement mode A or B or both. A PD indicates that it is standards-compliant by placing a 25 kΩ resistor between the powered pairs. If the PSE detects a resistance that is too high or too low (including a short circuit), no power is applied. This protects devices that do not support PoE. 


In effect, it's probably easiest to just use one of the several "Power over Ethernet ICs"; generally, google is your friend, but let's directly link you to  some common matching PSE product lines:


ST.com product category;
Texas Instruments product category;
Linear Technology's (now part of Analog Devices, Inc) product category, and one possible quick and dirty solution amongst many; [edited by My Other Head]


Note that PoE equipment typically is in the uncomfortable situation that they're trying to run power through communication lines. Since Ethernet uses differential signalling, you'll need typical ethernet signal transformers with a center tap on the network side if you want to use the cable for actual network, too:







simulate this circuit &ndash; Schematic created using CircuitLab

Now, if you don't actually plan on using the Ethernet as ... Ethernet a) you might be doing something strange and b) you might remove the magnetics (=transformers) and just terminate the ethernet lines with their specific impedance – 150Ω:







simulate this circuit

Note that the schematic above is but a dirty, inaccurate hack to make the ethernet port not see an open circuit at the end of your twisted pairs and interpret the port as unconnected.