There is no need to download anything, because while .Net 3.5 installer asks for downloading .Net3 files, in fact it has all the resources already available in it. Thanks to  @abbodi1406 for pointing this.

To install .Net framework completely offline:

1.       Before proceeding you may (or may not) have to install Windows XP service pack 2, Windows Imaging Component and Windows Installer 4.5 KB942288-v4. (I had them already installed)

2.       Install earlier versions of .Net Framework like .Net 2.0 sp2 (offline installers are available).

4.       Extract .Net framework 3.5 installer using 7-zip or similar program the widely available offline setup. It shouldn’t ask for internet connection now.

3.       Install everything from dotNetFX20 and dotNetFX30 folders.

4.       Install .Net framework 3.5 using main installer. It shouldn’t ask for internet connection now.

5.       You can test some .Net 3 programs. For example Microsoft Mathematics 4.0 which is a .Net 3.5 application.

Edited March 27, 20224 yr by TSNH
text formatting

Thank you so much for this tutorial, TSNH. It's very useful to me. 

What files are missing from the full .Net 3.5 offline installer?

 

you can build .NET 3.0 SP2 installer manually from dotnetfx35.exe

https://docs.microsoft.com/en-us/archive/blogs/astebner/creating-an-administrative-install-point-for-the-net-framework-2-0-sp1
https://docs.microsoft.com/en-us/archive/blogs/astebner/creating-an-administrative-install-point-for-the-net-framework-3-0-sp1

those guide are for SP1, but works the same for SP2

similar guide
https://docs.bmc.com/docs/ServerAutomation/89/deploying-net-653397308.html

FYI, standalone installer for .NET 3.0 SP2 actually exist, but it's not directly acquirable
you can find it within the VS 2008 bootstrapper packages
https://docs.microsoft.com/en-us/archive/blogs/astebner/vs-bootstrapper-packages-for-net-framework-2-0-sp2-and-3-0-sp2-available-for-download

click on Download location to get packages_setup.exe
then extract adtbs_sp2oob.cab/adtbs_sp2oob.msi
then use lessmsi to extract adtbs_sp2oob.msi

or install the whole package and find the installers under
C:\Program Files\BootstrapperPackages
 

1 hour ago, abbodi1406 said:

What files are missing from the full .Net 3.5 offline installer?

 

you can build .NET 3.0 SP2 installer manually from dotnetfx35.exe

It actually seems that no files are missing!  But why does it ask for downloading them if they are right there in the installer?

Edited January 25, 20224 yr by TSNH

I thought .NET 3.5 SP1's installer (dotnetfx35.exe, 231 MiB) contained .NET 3.0 SP2 already?

I know it includes files for 2.0 SP2, at the least: 3.5 SP1 (covering 1.0, 2.0, 3.0 and 3.5) and 4.0 (a separate component altogether) were available for XP.

Edited February 19, 20224 yr by Compa