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Go through the above article and its references for a clear understanding.
A hostid is most commonly the ID(MAC address) of your ethernet adapter. All ethernet(LAN) adapters that are manufactured are given a unique 48bits(6byte) address to uniquely identify it. https://en.wikipedia.org/wiki/Mac_address
On the other hand a hostname is any name you give your computer/device. Windows computer name can be changed from "My Computer" properties.
As for changing your IP address it depends on which network the adapter is connected to. Just read the above articles and you'll have a better understanding.
You can check your hostname information by typing ipconfig at your command prompt.