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The W5100S ethernet back-end now works end-to-end on real hardware: a complete, correctly-checksummed UDP datagram (10.0.0.99:40000 -> 10.0.0.50:6464, 32-byte payload) was transmitted by the FPGA-driven W5100S and captured on a PC, after a full bidirectional ARP handshake. Records the three W5100S-specific findings in CLAUDE.md (NETLCKR unlock, the 15-bit IDM address space putting the buffers at 0x4000/0x6000, and the 60.3 ms reset-to-ready time), and notes that UDP unicast is the reliable bring-up path while a 255.255.255.255 send sets SEND_OK without appearing on the wire. w5100_udptest.py is the self-contained configure+send+readback probe that isolated all of this. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>