Discover IP Space
Find the IP addresses that are related to your target.
Let’s say that you have a target and want to find the IP space for that target. By knowing this info you will have a good reference for how much they have out there on the internet.
The following sites will give you a good start to find infrastructures and hosts that owned by your target.
Also, you can try reverse lookup to discover other sites hosted on the server and get all A records linked with your target. This can be used to discover more environments owned by your target.
You can also use Shodan, the popular search engine for connected devices.
By using the above methods you can discover more top-level domains, IP addresses, subdomains, and more useful data that will help you in the recon process.