Understanding service enumeration

After finding out what systems are available and responding, the next step is to find out what services are available. Services such as email and web servers open ports for communication. The most straightforward way to find them is to perform a port scan.

Before we explore port scans, it is important to understand ports and how they work.

Introducing ports

There are a total of 65,535 TCP and 65,535 UDP ports available on any given system. It might seem that you would need to memorize all the ports; however, of these ports, the first 1024, also known as the well-known ports, are the ones commonly associated with specific services. An example of this is web servers, which commonly operate using port 80 on a system. A list of these port assignments can easily be found with a quick internet search; however, the Internet Assigned Numbers Authority (IANA) organization maintains the official list, which you can find at https://www.iana.org/. One...