Each segment of the filename provides critical information about the router's software capabilities: : Specifies the platform hardware, in this case, the Cisco 2691 Modular Router advipservicesk9 : Defines the feature set as Advanced IP Services
This appears to be a legacy release (12.4 is from the mid-2000s), no longer supported by Cisco for updates or security fixes. It is shown here for educational or archival purposes only. C2691-advipservicesk9-mz.124-17
: Indicates the image compression format. Each segment of the filename provides critical information