Could I clarify the data flow when a device within a network requests data from a device outside a network? So say I have a LAN, and I want to get data from a server outside my LAN. The server gets the IP address of the LAN's router from the frame, and sends the frame to the router based on that IP Address. Once the router receives the frame, the switch within the frame then uses the destination MAC Address within the frame to find the specific device within the LAN to receive the message. Is that right?
Please correct/clarify/elaborate on any of these points
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Been Googling without success sadly.
As I understand it at the moment, data passes down the OSI Model from Transport into Network into Datalink, IP Header is added with the Source/Destination IP Address, then Ethernet header is added with Source/Destination MAC address. This is based on either local ARP lookup or ARP discovery response. However, if the IP Address is not in the local network range the frame is sent to the Default gateway, assuming one is set.
So postulating a simple example - I am 192.168.0.1/24 and I want to message 192.168.2.2/24. As my application passes data to the TCP and on to IP then to Ethernet protocols, at some point something realizes that the destination IP is outside the local network, so this needs to go via the Default Gateway, which clearly has a different IP and MAC from the final destination device. I believe the IP address of the final device is added to the IP Headers, so how does the MAC of the default gateway get added to the Ethernet Frame headers please? Is it part of the functions of the Ethernet protocol layers (if so which one) or is it at the Physical Layer e.g. the NIC?
Can I ask at what point does the Default Gateway addressing get added to the Frame? I assume not at IP as the destination address must remain in the IP header to allow Routing? So is it at the Datalink layer or even a function of the Network Adapter/NIC at the Physical layer?
It should be basic level TCP question, still unclear to me. Let's say that two PCs with static IP communicate to each other via TCP/IP and both of them are behind their routers. And let's say that local PC already has (in its cache) remote PC's MAC address (so no need for ARP). If local PC wants to send something to remote one, it puts that MAC in appropriate data link layer field (target hardware address). But then, how that packet reaches local router at the first place, when there is no it's (router's) MAC address in appropriate THA field ?
Assuming there is no ARP cache in any nodes, and switch table is empty and IP addresses of source and destination are known (using DNS). Also all nodes are considered to be in same subnet. If n1 wants to send a packet to particular node say n2, it requires MAC address of n2.
I think in this case an ARP request will be send to get the corresponding MAC address. But my friend said that n1, will broadcast the main frame, and all nodes will extract the frame till network layer to know if the packet was for them or not.
Whether first method is implemented of the second will change the way table in switch is updated. So, can someone clarify my confusion?
The way it happens is that a host needing to resolve the layer-3 address to a layer-2 address will broadcast an ARP request. The host with that layer-3 address will respond with a unicast ARP reply to the requesting host.
A switch will see the layer-2 source address of any frame entering the switch, and it will build its MAC address table with the source MAC address and the interface where the frame entered the switch. Any unicast frame with a destination address which isn't in the MAC address table will be flooded to all other switch interfaces.
In the case you discuss, the first host will broadcast an ARP request, so the ARP request will go to all the hosts on the same VLAN. At the same time, the switch will add the MAC address of the requesting host to its MAC address table.
The target host will respond with a unicast frame to the requesting host. The switch, knowing on which interface the destination host is, by looking in its MAC address table, will send the frame directly to the destination host. It will also see the source address of the second host, and it will add that MAC address and interface to its MAC address table.
Then the original host can send its frames to the destination host, using the destination MAC address, and the switch will switch the frames directly to the destination host because it has the MAC address and interface in its MAC address table.
Initially the source node will send a broadcast request for the MAC address of the desired IP Address. The switch on realizing the coming packet is a broadcast, will channelize the packet to the other remaining ports. Switch will build the table for the corresponding source node.
All the other nodes will discard the packet, except the one for which it is. This node will reply its MAC address with a unicast reply.
Now since the destination of this packet is already present in the table of the switch, it will simply forward the packet to the corresponding port.
Network layer is responsible for defining the route a message takes from the sending host to the destination host. In particular, network layer decides what will be the target of the next hop on the way to the destination.
Data link layer is only concerned with transferring data between two nodes in the network. As I understand, data link layer must somehow get information from network layer - to which next node to forward the data.
My question is - how does data link layer get this information from network layer?
IP-header contains IPs of the source and the destination hosts. But what I am looking for is some field "IP of the next direct neighbour to forward data to"
Machines receive packets if the link layer address matches their hardware address. Usually the mac address is used at the link layer. The adapters will receive packets if the link layer is addressed to them. On receiving such a packet the network layer header of the packet is read by the system(O.S) to get the destination ip. From the destination ip, if the machine is an intelligent router, it will try to calculate the easiest path to the target(or sends to some default router), once this is found the link layer header is changed and will be set to match the hardware address of the next node in the path. However, the network layer header will not be changed.
"IP of the next direct neighbour to forward data to"
That is exactly what the link layer does, instead of ip it uses the mac address. And it is set by the operating system after reading the network layer.
You are looking for the Address Resolution Protocol (ARP).
First, your computer determines the IP of the next computer on the path (for example your local gateway (e.g. your DSL-Router)).
Your computer uses the ARP to identify the mac address corresponding to the IP.
The IP packet is sent to this mac address.
The Transport Layer sends segments of data to the network layer and the network layer creates a path to a destination based on the IP addresses and divides data into packets now data link layer makes frames based on these packets and the data is transferred now physically transferred from one destination to other based on MAC address
I recently found that packets are encapsulated within ethernet frames. Packets use IP addresses, frames use MAC addresses.
Why aren't IP addresses used in ethernet frames for routing? I understand that when trying to access a basic website, the computer goes to a DNS to find the IP address relevant to the user-entered domain name. How do computers find the correct MAC address?
Really, how are MAC addresses used in routing internet traffic?
Thanks
IP packets aren't always encapsulated in Ethernet frames. There are other physical media such as ISDN, etc. When packets are routed, IP addresses are used to determine the next hop and the physical address is used to physically identify the interface serving as the next hop. Only the former (determining next-hop) is usually called routing.
To answer your second part, MAC addresses are discovered through ARP (Address Resolution Protocol) in IPv4 & ND6 (Neighbor Discovery) in IPv6.
Update:
The destination IP address in the IP header is the final destination. In the process of routing (at each hop), you get the next hop's IP address to (eventually) reach the final destination from the routing table (this could be a default gateway's IP address). To send the packet to the next hop, you need its MAC address. While hopping through intermediate links, the IP address in the IP header don't change - only the MAC addresses change.
Bit late but still here is my answer :) ...
To send data you need two address, the MAC address and the IP address.
Basically the sending host will ARP for a MAC address, this occurs when the local host doesn't know the MAC address of the host it has an IP address for or it will ARP for the default gateway MAC address (if it doesn't already know it) if the IP address in on a different subnet/ network. Once it obtains a MAC address the IP packet is encapsulated in a L2 frame and sent across the media. If the IP packet is meant for a host on a different subnet/ network, it will be sent to the default gateway, this router will de-encapsulate the L2 frame (remove and discard it) check the IP address and will forward it. For the router to do this it needs a MAC address to send it over the media, It will look up the next hop in it's routing table, encapsulate the IP packet with the same source and destination IP address that was sent from the original host into a new L2 frame. This time the MAC address for the source address will be that of the forwarding interface of the router, and the receiving interface of the next hop will be the destination MAC address. This will continue from hop to hop until it reaches the final host, each time the MAC addresses will change, but the original IP address will remain the same.
Here's the key point -- there can be more types of packets than INTERNET traffic. You could be using IPX, which is non-routable. How do clients identify each other? By the MAC address.
Routing != Addressing, which is really where the MAC comes into play.
In order to be routed, the OSI model adds a layer to allow for path discovery to the next gateway. This layer is responsible for routing, but knows nothing about the MAC address.
As a side note, at the hardware level, MAC addresses ARE used by switches, but not for routing. From How Stuff Works:
The switch gets the first packet of data from Node A. It reads the MAC
address and saves it to the lookup table for Segment A. The switch now
knows where to find Node A anytime a packet is addressed to it. This
process is called learning.
In this way, a switch can make sure that traffic is only outputted to the correct port. This isn't accomplishing routing so much as reducing network congestion. Only broadcasts and traffic destined specifically for that MAC address should be sent out the port.
Recently I have been thinking about the same and came upon this question. Here is my answer to this question. Actually MAC address is needed for correctly sending the packet to right destination. This is specially true when packet is needed to sent over a VLAN. There can be multiple switches/routes connected on that VLAN over multiple physical interfaces. However IP Routing is unaware of these physical interface. It only knows about the logical connectivity. For example, route 10.10.10.0/24 is reachable via VE/VIF0.10(logical VLAN interface) and/or nexthop neighbor is 20.20.20.1. There could be multiple interfaces under VLAN 10. Then to which interface packet is sent out? This is where ARP comes in the picture. ARP helps to discover the MAC address associated with the next-hop IP address. When switch/router learns the nexthop MAC. along with that it learns the physical interface also via which that MAC is reachable. Hence while routing packet, firstly MAC corresponding to the destination IP is searched and then the physical interface associated with that MAC is searched. Finally packet is sent out via that physical interface. The MAC corresponding to that destination IP is used as destination MAC. In absence of this, routed packets will always be flooded in the outgoing VLAN.
Hope this helps.
Thanks.
Answer: MAC addresses are not used in the process of routing of a packet.
segment -> transport layer (TCP ports)
packets -> network layer (IP addresses)
frame -> data link layer (MAC addresses)
bits -> physical layer (electric/optical signals)
Create your own packet/segment visit http://wirefloss.com/wireit/
There are 2 models (TCP/IP and ISO/OSI)
In detail:
Your app has some data. This is encapsulated by mentioned layers. Encapsulation means that a header with fields is added at each layer. If your data never leave the local network the MAC address will be the same. Once your data needs to be delivered outside your network the frame header is stripped by router and is replaced by router fields.
UPDATE 2021: Some people seems never heard of ISO OSI model and put this answer as incorrect.