DNS Searches

Very comprehensive site to search for domain/IP registrations.  What domains are within specific address blocks etc… Easy to copy and paste into a spreadsheet for handling.

http://ipv4info.com/

A great site to lookup and traverse DNS.  Will provide websites associated with domain, mx records, other subdomains of domain etc.

https://www.robtex.com/dns-lookup/

A wonderful site to skim subdomains and additional information and tools

https://dnsdumpster.com/

Another great subdomain tool

https://findsubdomains.com/

Online Dig

https://www.diggui.com

https://toolbox.googleapps.com/apps/dig/