: This is likely a version or build identifier, often including a release date (May 2018 in this case). : The standard file extension for a executable image.
The filename refers to a specific Cisco IOL (IOS on Linux) image, a vital tool for network engineers practicing in virtual environments like GNS3 or EVE-NG . Understanding the IOL Image i86bilinuxl3adventerprisek9m21573may2018bin
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Understanding i86bilinuxl3adventerprisek9m21573may2018bin: The Backbone of Virtual Labs : This is likely a version or build