Microsoft .net Framework 4.8.1 Offline Installer Free -

If the installer won't start, check Windows Update. Sometimes a pending "Security Intelligence Update" can lock the installation service.

Microsoft .NET Framework 4.8.1 Offline Installer: Everything You Need to Know microsoft .net framework 4.8.1 offline installer

If you are an IT admin managing 50 computers, downloading the 100MB+ file once is much more efficient than having 50 machines download it individually. If the installer won't start, check Windows Update