Abhishek,
I would recommend you check out Rene’s lesson on QoS Classification. In this lesson, you will discover that there are all kinds of tools at your disposal to be able to identify, classify, and mark traffic for QoS treatment throughout your network. It is rarely the case, for the reasons you brought up, that a simplistic approach of using just IP addresses is used.
As for your second question, you are essentially asking about what QoS is meant to do as a whole (so there isn’t a quick, easy answer). The key to getting QoS working properly relies on the network engineer doing a few critical things before actually configuring anything:
- Work with your business units to learn network expectations, and what applications are important to them
- Determine a kind of SLA from the business units to gauge how they want their applications to perform under heavy load
- Perform an “inventory” of what kind of traffic is present on your network, as well as the patterns of the traffic. For example, does it spike at certain times of the day? How do these spike times line up with how the rest of the business is using the network?
After you have answered questions like these, then the actual QoS work begins. In a nutshell, you need to identify and mark important kinds of traffic as close to the source as possible. Once the traffic is marked, you can configure the devices on your network to treat this traffic in a way that is consistent with the expectations you learned from your meetings with the business units.
As in example in your case, you could use NBAR or a combination of IPs and ports to classify the Skype traffic. You would have this traffic assigned a high priority marking (like DSCP EF, for example for voice data), then configure queuing (perhaps LLQ) to ensure this traffic is processed quickly under load.
As you can see, QoS is a complicated topic–it will take a lot of reading and study on your part to master it!