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between devices on the Internet, much like making a phone call. Packets can be sent back and forth between both devices, and each one will tell the other to resend any packets that were lost or had errors, just like you can tell a friend to repeat what they said if you couldn't hear them on a bad mobile phone call connection.

You'll also hear these terms used with IP:

3. ICMP (Internet Control Message Protocol) - these packets/message are typically generated in response to errors in IP transmissions (defined in RFC-1122) or for diagnostic or routing purposes. The 'ping' utility, which can tell if a connection between two devices as actually working, uses this protocol. Note that just because you can 'ping' a remote device does not mean you can make a successful network connection with it using UDP or TCP. It just means the connection is working over the network and the remote device is turned on and working.

4. ARP (Address Resolution Protocol) - a way to take the 48-bit unique number, known as a MAC (Media Access Control) that is assigned to every network device, and "map" it back to an IP Address. As an example, a network card in your computer might have a permanent MAC of "00-08-47-4C-1F-2D" and a program running on a system can be configured to associate that with a particular IP Address, such as "192.168.1.3".

5. RARP (Reverse ARP) - simply does the reverse of ARP, by mapping a unique IP Address back to a unique MAC.

In the definition of the Internet Protocol "Suite Stack", also known as the "TCP/IP Stack" (just to make things confusing) there are four major layers defined from the hardware itself (Level 1) to an application running on a system such as a Web Browser (Level 4). They are as follows:

Level 1: Hardware - Ethernet, Token Ring, Wi-Fi, ATM, PPP, SLIP, Frame Relay, etc.

Level 2: Internet - in a circular fashion, this is where the IP itself runs, along with ARP/RARP, and routing protocols which tell network devices where to send each packet.

Level 3: Transport - this is where the UDP or TCP connection is made.

Level 4: Application - this is the program communicating over the network, such as a Web Browser or your Email Client.

Most of the hardware on Level 1 is well-defined and made, and applications on Level 4 are well-specified as well, but the code that handles moving the information from Level 1 back-and-forth to Level 4, which is known as the "stack", has to be carefully written to be extremely efficient and able to handle slow connections, lost packets (in the case of TCP connections), errors in packets, etc.

So to sum it up, "IP" actually means many things, and can be very confusing, but it is basically the protocol that defines how devices over a network, such as the Internet, identify themselves, find each other, and communicate by sending information in "packets".

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