mTCP is small enough to be used easily on DOS systems all the way back to the original PC. I have several XT class systems which I have restored. I have a few 8-bit ISA NICs which I use along with the FTP server provided with mTCP to transfer files to them.
mTCP, still supported and accompanied by a handful of utilities (including an IRC client), and the venerable
Waterloo TCP (WATTCP) developed in the early 1990s."