THANKS!!!...see my response below in your quote.
I'll respond in line to your questions:
Layer 1 consists of the "physical" stuff-- the things you could touch and manipulate, like wires, and the specifications of how the data is transferred, like +5v for 1, +0v for a 0.
I don't understand the +5v for 1 and +-v for a 0 thing, please explain that a little.
when you're talking about signalling, you've got a base voltage, let's say it's 5 volts. (I forget offhand what ethernet uses). A 0 would equal 5 volts on the line, and a 1 would equal +5v, so it would equal 10volts. That way, if it pulses 5 volts, the NIC interprets that as 0, if it's 10 volts, it interprets that as a 1. The voltages can be arbitrary, but I think they were chosen for a specific reason. What reason? I'm not sure.
Layer 2 deals with MAC addressing, mainly. ARP is a tool that translates IP addresses to MAC addresses, and RARP does the opposite.
I was reading up on MAC addressing and from my understanding its kind of like a burnt in address for hardware so hardware can be detected on a network. Am I right or way off? Look at this:
"ARP/RARP is commonly used to map the layer 2 MAC address to an address in a layer 3 protocol such as Internet Protocol (IP). On broadcast networks such as Ethernet the MAC address allows each host to be uniquely identified and allows frames to be marked for specific hosts."
What do they mean my "map"??? How is the mapping done? And why did they call Ethernet a "broadcast network"?
You are correct-- MAC addresses are burnt onto the hardware, but they can also be "spoofed", ie, changed, by software. Normally, though, it is burnt in. "Mapping" Just means "translating", or "look up"-- each host carries its own ARP table, and it either performs the lookup for host-> IP address there. If it's not located there, it broadcasts out, "WHO HAS THIS IP ADDRESS?" to the network, and the host with that IP responds with its MAC.
When you say it's a "broadcast" network, that's all you can say-- one host contacts the other essentially by saying, "I WANT TO TALK TO MAC ADDRESS XX
X
X
X
X
X", and that host responds to the system.
Layer 3 is IP, mainly. That's where your IP address comes into play, and that's where routing functions take place at. Again, IP addresses mean nothing if they don't correlate back to a MAC address on Layer 2.
Okay so layer 2 to translates the MAC Address of the hardware to IP addresses so routing of data can take place at layer 3 of a network. Am I right about that?
Sort of. ARP kind of "spans" the layers-- it's a table that keeps up with the level 2 MAC address, and correlates that to the Layer 3 IP address. Routing is all done at Layer 3. Routing tables accomplish this, by saying, "I know this IP address is accessible through this other router 1.1.1.1" Usually, there's a "default gateway", that unknown IP addresses are routed to.
Layer 4 is TCP, or UDP, or ICMP, even. This specifies what "port" to listen on for a particular service. I equate this out to being in an Office building-- you've arrived at the building (the IP address), but you still need to specify what office you're going to (the TCP port number)
I was reading up on TCP, UDP and ICMP last night in my Net+ book and I remember it mentioning the ports. I like your analogy, makes much more sense than that book
Thanks. The Cisco curriculum didn't make any sense to me. As a matter of fact, the concept of ports didn't make sense to me until I started working with multiple services on a system, and I started messing around with Port forwarding and IP Masquerading on routers. You'll run into that fun later, probably
Layer 5 deals with entering, leaving, and staying in that office. "Handshaking" is a common term to see in this layer.
Isn't a handshake an "Ping"???...im a little lost at this point but i think was reading up on this last night as well, a session is like a line of communcations that is created so computers/hardware can "talk. Am I right about that?
Handshaking isn't really like a ping. A ping would look like this:
System 1: I'm up, system 2, are you there?
System 2: Yeah, I'm up.
Where a handshake basically looks like this:
System 1: I'm ready to send data to you... you ready?
System 2: Yeah, I'm ready-- you ready?
System 1: Oh, I'm
SO ready. Here it comes!
kind of silly, but it gets the job done
Layer 6 takes the data we've recieved from the lower layers, and formats it properly for the requesting application. This deals with things like character sets (ASCII, EBCIDIC, Unicode, etc)
Is this a software thing or a hardware thing? I guess my question is WHAT formats the data and how? I need to be able to picture it happening so i'll fully understand
Really, it could be software, or hardware-- it depends on where it's implemented in the system. (Kind of like how you could have an MPEG decoder card, or you could just have a software codec to do it). Usually, it's done in software.
Typically, this doesn't get mentioned much, because we usually work in ASCII. Look up
Character sets to get more information on it-- ASCII uses a certain number of bits to represent a character, EBCIDIC uses another amount, and Unicode uses an entirely different amount of characters to represent that same character. This is where, as I understand it, that translation takes place.
Layer 7 is where all of the data is actually worked with. The previous layers were just there for the data to reach the final endpoint of the application. This can be anything from a web browser, a MUD client, or a database server application.
hmmmm, i was thinking the application layer dealt with software applications, but after reading that i think it deals with the actual application of data and not software applications. I'm a little confused at this point
There's always the chance that I'm slightly off. I think that a more precise positioning of the Application layer is at the very end of the process, when the TCP/IP stack hands the data to the actual requesting application. My instructor (keep in mind, this was like, 2-3 years ago) didn't really make too big of a distinction. Basically, I look at the Application layer as the "glue" or "pipe" that actually connects the application to the network.
...That's how I've always understood it. That understanding was good enough to get me a degree in this stuff, at least.
^^^hahahahaha, i hear you, i have a b.s. in Information Systems, but I didn't take not one Networking class, it wasnt offered. I am trying to prepare for the A+ and Net+ exam myself. It gets very confusing with no visuals, digrams and a whole lot of acronyms. Right now I've read about half the Certfication book and don't have a clue on how to build a network. I just have protocals and definitions
Trust me, it makes a lot more sense once you start plugging in wires. Also, keep in mind that the OSI model is not the actual model used these days-- the TCP/IP stack is what's used for this. The TCP/IP stack is much more simple:
http://www.pku.edu.cn/academic/resea...ml/TC0102.html
There's only 4 layers:
- Application
- Transport
- Internetwork
- Network access layer
In that one, network access layer is the wires and MAC addressing type of stuff. Internetwork is IP addressing and routing. Transport is the TCP/UDP ports. The Application layer is essentially "everything else"-- where you actually "do stuff with the data". Much more simple, because you don't have to deal with the presentation layer, which is often done implicitly.
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