atcomsystems.ca/forum Forums VOIP Phones & IP PBX Networking Data packet structure

So I was taking a test the other day on some IT stuff. One of the questions was how is a data packet delivered from point A to point B. Two of the 4 answers didn't make any sense at all.. The other two were from IP address to IP address, or from MAC address to MAC address. I put the IP address. I believe the correct answer is MAC address. But why MAC address and not IP address?

If they used the term data "packets" then I would have an impulse to enter IP address, as the usual term for data on the network layer is called packets, while the MAC address is usually called "frames".

I would think a lot would be in the wording, but I think in general the Data Link layer, IE MAC address is resposible for moving the data, while the Network layer (IP address) handles the routing, to figure out how to get there.

The IP address is translated to a MAC address using ARP. The devices will use the MAC address of the remote PC to send the data across the wire.

If the other device is not on the same network, then the data is sent to the router which will find out how to pass it on. In this case the PC will send data using the MAC of the router.

MACs are passed at the datalink layer also known as layer 2 of the OSI model. Layer 2 guarantees error free transmission using FCS. Only matching MACs will acept data frames. As such it is interface to interface. IP works at layer 3 of the OSI model. It uses a hierarchical transport, TCP, where the IP is translated to the MAC within an encapsulated portion of the packet. IP packets can be accepted by any layer 3 device and forwarded which makes them routable.

It is the 7 layer OSI model: physical, datalink, network, transport, session, presentation and application.

Yeah, it boils down to the OSI model. Layer 2-4 are the ones that usually confuse me (and others it seems). My over-simplified approach is as follows (with examples):

Layer 1 - The cable (cat-7 preferably :p )
Layer 2 - The link light (Ethernet)
Layer 3 - The network address (IP)
Layer 4 - The protocol (UDP/TCP)
Layer 5 - Protocol Control Mechanism (http)
Layer 6 - The OS (Windows)
Layer 7 - The Application (firefox)

That's how I remember them. Then there are things that don't really adhere that well to the model, like TCP for instance. It's a mix of various portions of the model (3, 4, and 5 in this case). Basically, it helps you diagnose broken networks really. Every higher layer depends upon the lower layers to work. So if the network isn't working, start from layer 1 and work your way up.

And from my experience, it all gets kind of blurry past layer-4 anyways. So I probably wouldn't worry much about 5 through 7 (the upper layers) unless you plan to be a programmer/developer.

Thanks for all the information guys. This is really helpful.

Good info here.
Just a quick add from an IT dummy. When we configure IP phones or add them--the system seems to know what all the MAC addresses are (I can see them when the tech boots up the phone on site) but then, in order for the phone to connect correctly, he types in the correct IP addresses--boot server & such. The only thing I use the MAC addresses for is to make sure that they are the same on the phone and in my programming screen. That's all I know.

IP phones use MAC addresses almost like a serial number, since the extension is tied to that phone and not a port in the system. IP systems can configure a specific phone based on that MAC address.

As far as network configuration, most people are aware of IP address information, as this is something that they have to set, or check. MAC is known and used by all the other devices on the network (at least within a broadcast domain).

I don't want to overdo this thread, but I like to translate tech to other things (like I visualize electricity as water through a hose).

This is the best I can come up with:

Mailcall:
Captain gateway has a letter addressed to Private 192.168.1.100. The subnet on the envelop tells him that Private 192.168.1.100 is local to him.
He uses his ARP loud speaker to yell "Where is private 192.1681.100?"
From the corner a replay say "Here I am. My MAC ID is 00:11:22:33:44:55:66!"
Captain matches the MAC for private 192.168.1.100 and hands him his letter. The Private will make sure that the MAC on the letter matches his and will reply with a thank you ACK.

Before switches, the Ethernet connections were share with all devices. Everything was broadcasted on the netork for all devices to hear. If the message was not a broadcast message or if the MAC did not match, the PCs would ignore the message.

And please anyone feel free to Critique or correct my analogy.

Actually, a MAC *IS* a serial number. The first 3 octets identify the manufacturer/model/etc of the device, and the last 3 are the actual serial numbers for the device. Think of it like VIN codes on car's but for network gear.

Here, put in the first 3 octets (6-characters) of your network devices here: https://db.uga.edu/network/public/vendorcode.cgi

That is true. I still think of it as "like" a serial number because some of the equipment I deal with, especially with hybrid PBXs have both a serial number and a MAC address.

For the full IP systems or IP phones we usually just use the MAC address. Nowdays applications are licensed based on the MAC of the NIC running the software.

That is a usefull link, but I like using Netscan, which after scanning the MACs on the network, it will show the vendor info for all NICs on the network. Sometimes his helps isolate a device you are looking for in a network.