Yes, Benjamin. When using fastboot with a device connected over TCP/IP, you can specify the serial number (or TCP/IP address) in a few ways:

1. Command-Line Argument

The most common way is to use the -s flag directly in the command:

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fastboot -s tcp:IP:PORT devices

or

bash
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fastboot -s tcp:192.168.1.100:5555 flash boot.img

2. Environment Variable ANDROID_SERIAL

You can also set the ANDROID_SERIAL environment variable to specify the target device. This works for both USB and TCP/IP connections:

bash
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export ANDROID_SERIAL=tcp:192.168.1.100:5555
fastboot devices

or

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ANDROID_SERIAL=tcp:192.168.1.100:5555 fastboot flash boot.img

This avoids repeating -s tcp:IP:PORT in every command.

3. Using adb for TCP/IP Mode

If your device is already connected via adb over TCP/IP, you can use:

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adb tcpip 5555
adb connect 192.168.1.100:5555
fastboot -s tcp:192.168.1.100:5555 ...

But this is more about adb than fastboot.

4. Config Files (Limited Support)

Some custom ROM tools or scripts may allow you to set a default serial in a config file, but this is not a standard feature of fastboot itself.

Summary Table

Note: The ANDROID_SERIAL environment variable is the closest alternative to the -s flag and is widely supported.