Ncat operates in one of two primary modes: connect mode and listen mode. Other modes, such as the HTTP proxy server, act as special cases of these two. In connect mode, Ncat works as a client. In listen mode it is a server.

In connect mode, the <hostname> and <port> arguments tell what to connect to. <hostname> is required, and may be a hostname or IP address. If <port> is supplied, it must be a decimal port number. If omitted, it defaults to 31337.

In listen mode, <hostname> and <port> control the address the server will bind to. Both arguments are optional in listen mode. If <hostname> is omitted, it defaults to listening on all available addresses over IPv4 and IPv6. If <port> is omitted, it defaults to 31337.