How do I distribute a large download over multiple computers?

The common protocols HTTP, FTP and SFTP support range requests, so you can request part of a file. Note that this also requires server support, so it might or might not work in practice.

You can use curl and the -r or --range option to specify the range and eventually just catting the files together. Example:

curl -r 0-104857600         -o distro1.iso 'http://files.cdn/distro.iso'
curl -r 104857601-209715200 -o distro2.iso 'http://files.cdn/distro.iso'
[…]

And eventually when you gathered the individual parts you concatenate them:

cat distro* > distro.iso

You can get further information about the file, including its size with the --head option:

curl --head 'http://files.cdn/distro.iso'

You can retrieve the last chunk with an open range:

curl -r 604887601- -o distro9.iso 'http://files.cdn/distro.iso'

Read the curl man page for more options and explanations.

You can further leverage ssh and tmux to ease running and keeping track of the downloads on multiple servers.