Communicating with ports

Question

I have a question about developing code that can communicate to a port and therefore someting in the outside world. I see tutorials for doing this for various things but I don't understand how they know what to assign for different "packets" that they create when sending out data or receiving data, for example: (from IBM's "intro to gps"...),
byte[] request = new byte[6];
request[0] = 0x10; // DLE
request[1] = (byte) 0xfe; // Packet Id
request[2] = 0x00; // size of packet payload (there is none!)
request[3] = (byte) (0x100 - request[1]); // checksum
request[4] = 0x10; // DLE
request[5] = 0x03; // ETX
try
os.write(request);
os.flush();
I can kind of grasp the concept of creating a "packet", but how does one get the information so they know what to assign to each section of the packet like:
request[0] = 0x10 or something of that nature. What are the methods someone has to take in order to successfully "communicate" to another device, regardless of what that device is?

Answer 1

(Try that again with the italics correct.)
I have a question about developing code that can
communicate to a port and therefore someting in the
outside world. I see tutorials for doing this for
various things but I don't understand how they know
what to assign for different "packets" that they
create when sending out data or receiving data, for
example: (from IBM's "intro to gps"...),This is a protocol. From www.geek.com....
Protocol - This is the behavior that computers must follow in order to understand one another. Think of it as a language. If two computers don't use the same network protocol, then they cannot communicate.
Notice in the above it doesn't say anything about sockets, IP, TCP/IP or even really networks (network is provided as an example only.)
As the definition says a protocol specifies how the computers communicate. What you want to know is the definition for the protocol. The definition, should tell you what all the values are. Unfortunately not all protocols are complete.
The definition will be a document that you get from...somewhere. And the somewhere is hard to say. For internet protocols you can find definitions in RFCs from www.ietf.org. Other protocol documents come from other places - and some of those places you will have to pay money to get the document.
I can kind of grasp the concept of creating a "packet", Keep in mind that if this protocol moves over an TCP/IP (or IP) protocol that that means it is a protocol on top of another protocol. And IP and TCP/IP both have packets (they are protocols within protocols) and so the protocol that you are looking at will be on top of the other one.