Internet-Security.com TCP port 1604 details

TCP port 1604 is used by the Citrix ICA Browser service in legacy Citrix environments (e.g., Citrix WinFrame, MetaFrame 1.8/XP, Presentation Server 3.0/4.0/4.5, and XenApp 5).
This service handles server/app discovery and browser elections for older Citrix Program Neighborhood clients and many legacy thin clients, before the actual ICA session is established on 1494/TCP (or 2598/TCP with Session Reliability).
In real deployments, Windows servers running Citrix MetaFrame/Presentation Server/XenApp would listen on 1604/TCP (and 1604/UDP) to let clients enumerate published applications in a farm.
Modern Citrix stacks (StoreFront/Workspace app) typically disable this in favor of the Citrix XML Service and do not rely on 1604/TCP.
Historically, exposing the ICA Browser service externally has enabled enumeration of Citrix servers/apps, so best practice is to restrict or block this port.

TCP 1604