In legitimate networking software, the term portable typically means “runs from USB without installation.” Cisco does produce portable IOS images. When you see portable appended to an otherwise credible-sounding filename, it almost always indicates:
: i86bi-linux indicates it is a 32-bit x86 binary designed to run on a Linux host (IOU). i86bilinuxl3adventerprisek9m21573may2018bin portable
While there isn't a single "story" in the traditional sense, the string i86bi-linux-l3-adventerprisek9-m-215.73-may2018.bin In legitimate networking software
Let's break down what each part might signify: i86bilinuxl3adventerprisek9m21573may2018bin portable