: This part of the filename represents the IOS version.
For anyone building a CCNP or CCIE home lab using GNS3 or actual 3700 series hardware, this specific IOS version is a workhorse. c3725-adventerprisek9-mz.124-25d.bin
The filename can be broken down to understand exactly what it offers: : Designed for the Cisco 3725 modular router platform. : This part of the filename represents the IOS version
conf t boot system flash:c3725-adventerprisek9-mz.124-25d.bin exit write memory c3725-adventerprisek9-mz.124-25d.bin