While building a particular packet to be sent across or outside the network, the host, at a certain point, needs to specify the MAC address of the interface through which the packet is going to be sent. To know this address, the host checks its routing table for a network that includes the IP address of the destination node and retrieves the IP address of the interface through which packets to that network are supposed to be sent. My question is concerning the step that happens after this. How does the host translate the IP address of one of its interfaces to the corresponding MAC address? Does it check its MAC address table or are these correspondances stored in some global data structure that the host can access independently of its MAC address table?
Routing MAC Address Interface – Where Does a Host Save Its IP Address?
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Good question. I'll answer it with an animation:
When Host A sends the frame, the switch does not have anything in its MAC address table. Upon receiving the frame, it records Host A's MAC Address to Switch Port mapping. Since it doesn't know where the destination MAC address is, it floods the frame out all ports.
This assures that if host B exists (which at this point, the switch does not know yet), that it will receive it. Hopefully, upon receiving the frame, Host B will generate a response frame, which will allow the Switch to learn the MAC address mapping from the return frame.
You can read more about how a Switch works here (where I took the animation from). I would also suggest reading the entire article series for a closer look at how a packet moves through a network.
One last note regarding the terms Flooding vs Broadcast. A switch never broadcasts frames, a broadcast is not an action a switch can take. A switch can only flood a frame. A broadcast is simply a frame with a destination MAC address of ffff.ffff.ffff. This is often confused because the end effect is the same, but they are actually different.
When you set Router Interface as next hop for a Network, the router assumes that the Network is directly connected to that Interface. In your example, Router A will assume that the Network is directly connected to Egress interface, there is no concept of Router B. It will send out ARP requests for each destination, and when it receives a response back, it will forward the packet out based on the response it gets back.
Now, how does that work. Router B needs to be configured to supportProxy ARP. It will respond to ARP requests from Router A, and will reply back with the MAC address of its ingress interface.
Best Answer
Every operating system is potentially completely different and may do more than one thing in regard to this question. The operating system may store its various interface MAC addresses at some location in a configuration file on disk, or simply store it in RAM on start up, or in a database of some kind. It is up to the operating system to do so.
Or it may not store it at all and may simply rely on the network interface driver software to inject such information in packets/frames as needed when they are encapsulated and put 'on the wire'. Technically the operating system (and certainly the application on a device) does not need to 'know' the MAC address of a given network interface in order to send or receive data over it. The interface and its driver software do need to have that in their configuration. This kind of abstraction is key to the layered network model where each layer only needs to deal with its own function and can rely on its neighboring layers to perform their functions.
Of course, some applications may want to access information about IP addresses, MAC addresses, ARP or ND tables, etc. so that kind of information can be accessed for tools like configuration interfaces, traffic capture tools, command line tools and display, etc. But those are diagnostic applications that specifically need to access that data as part of their function. An email program, for example, can send an email without knowing the MAC address of the network interface it uses to make an SMTP connection.
You shouldn't think of the computer as a wholistic organism that has to understand its own working components to function optimally. It is a collection of connected subsystems that work together to accomplish complex tasks that are broken down into tiny mathematical functions and only look like a familiar job to us from the top down view we have as human beings.