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ICQ & AIM
Port 5190 is widely recognized as the default communication port for ICQ and AOL Instant Messenger (AIM), which were among the earliest widely adopted instant messaging platforms. These services enabled real-time communication over the Internet using proprietary protocols that primarily operated over TCP. This port facilitated user sign-in, status updates, message exchanges, and file transfers between IM clients..
Port 5190 was extensively used by the proprietary protocols of ICQ and AOL Instant Messenger, known as OSCAR (Open System for Communication in Realtime). When a user launched an IM client, it initiated a TCP connection via this port to the service provider’s centralized servers to establish presence and facilitate real-time message exchanges. The communication over this port enabled user sign-ons, contact list management, presence updates, and transmission of chat messages.
Beyond basic messaging, this port also handled ancillary functions, including file transfers, offline messaging, and chat invitations. Since the protocol was closed, third-party software had to engage in protocol reverse-engineering to achieve interoperability. Over time, the use cases expanded to include features like chat rooms and multimedia integration over the same port.
Despite shifts to newer messaging paradigms and platforms, port 5190 serves as a notable historical reference for instant messaging technology. It exemplified the early architecture of centralized messaging platforms relying heavily on a single TCP port for all primary communication functions.