The hardware architecture of the Cisco 2800 Series Integrated Services Routers (ISR G1), which uses the prefix, reached its final software ceiling at Cisco IOS Release 15.1(4)M 🔍 Breakdown of the File Name

Do not download IOS images from third-party sites (e.g., rapidshare, file-sharing forums). They often contain malware, have illegal MD5 checksums, and may cause bricked routers.