ADB: missing port in specification

I can able to connect my android phone by adding a port number in the postfix place.

For example,

$ adb kill-server

$ adb connect 192.168.1.20:5555

Directly it could not able to connect, it was giving me the above error.

NOTE: Port number is compulsory to apply, maybe your port number is currently different so please find that and then try to connect again.

Update

This bug has now been fixed as of ADB version 1.0.41, which is part of Platform Tools version 29.0.4. The fix for the bug was committed on 31st July 2019:

Restore default port for adb connect.

The default port of 5555 was removed a while back, but the help text was never updated, and other contexts still allow a default port.

Bug: https://issuetracker.google.com/128561172

Inputting adb connect 192.168.1.20 without the trailing port number now results in ADB connecting to the target device, restoring previous behaviour.

Old answer

This would appear to be a bug within ADB, introduced in December 2018 or January 2019. I believe this relates to recent changes to this else statement in socket_spec.cpp.

} else {
    std::string addr(spec.substr(4));
    port_value = -1;

    // FIXME: ParseNetAddress rejects port 0. This currently doesn't hurt, because listening
    //        on an address that isn't 'localhost' is unsupported.
    if (!android::base::ParseNetAddress(addr, &hostname_value, &port_value, serial, error)) {
        return false;
    }

    if (port_value == -1) {
        *error = "missing port in specification: ";
        *error += spec;
        return false;
    }
}

If a port value is not specified, the variable port_value is initialised at -1 and does not change. This value is not altered by android::base::ParseNetAddress either. If the ParseNetAddress check passes then we will always fall into the error-catching statement immediately afterwards.

$ adb kill-server
$ adb connect 192.168.1.20

Just kill the adb server and connect normally. Default port (5555) is restored when server is restarted.