Port 6600
In live entertainment systems, port 6600 is used by HogNet.
udp/6600
HogNet — Default show/session identification port; additional shows increment from 6600 (6601, 6602, ...)
High End Systems' proprietary Ethernet network for linking Hog consoles, DPs and nodes, on a dedicated NIC/connector separate from the FixtureNet connection Hog consoles use for outbound Art-Net/sACN to fixtures and gateways. High End Systems (now part of ETC) has confirmed on the record that HogNet is undocumented outside their own equipment.
Check it from the command line
npx showstack port 6600
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If you know something else uses port 6600, tell us. Adding it is one file.