Regular information technology or IT protocols are are the protocols that are in use on everyday IT networks. Some examples of those protocols include HTTP, HTTPS, SMTP, FTP, TFTP, SMB, and SNMP. This doesn't mean these protocols are used exclusively for IT purposes. Many OT devices, for example, will incorporate a diagnostic web page or use FTP to receive an application or firmware updates:

Traditionally, these protocols were used only outside of the plant floor and ICS networks in levels 4 and 5 of the Purdue model. With the trend of converging OT and IT networks technologies, many of these protocols can now be found all the way down to level 1 and with them their vulnerabilities too, which have plagued regular IT networks for years.