Hello, i just did a quick review on my CCNA material and it has risen me a few questions:
1)I know you have IP and the 3rd OSI layer - but why do we really need it? we have unique MAC addresses for every NIC so that should be enough, so why do we need IP after all?
2)Same goes for subnets - why do we need them? what is their main purpose?
3)Suppose i have some basic Router topology going on - i mean, i have set it all up but didn't turn it on and let it work. so at the moment i do let it work suppose some end-point computer wants to send data to another computer that is on a different subnet\LAN - how does the router acquire the information needed for his route-table. i dont quite agree with route-updates(in case the routing protocol is distance vector) because as i understand it send UPDATES but there is nothing to update because first the information should be acquired..(and what about link-state and hybrid?)
1.) Computer A trys to communicate with a Computer B not on it's subnet.
2.) Computer A sends a packet to it's default gateway
3.) The gateway then determines how to get to Computer B (routing)
4.) At the destination network, the switch arps for the mac address associated with the desination IP address in the packet.
5.) Computer B reverses the process to send back to Computer A
Question 1 should be answered by my answer 3.
Question 2: Subnets conserve IP address space. For a point to point connection, why waste an entire classfull network (/24) when you can use just a portion of that classfull network?
Edit: I'm not sure if you are asking how routing takes place, but if you are I guess I failed to answer it so I will attempt to do so now.
Routing is based on, you guessed it, routes. These can be manually configured or dynamically learned. You might think of basic routing like you would a DNS hierarchy.
ip route network1/subnet via interface 1
ip route network1/subnet via interface 2
ip route 0.0.0.0 0.0.0.0 via interface 3
In this example, the router knows about two directly connected networks that it can route to. The last line is a "catch-all" which basically says, if all configured/learned routes are no good for the destination I am trying to reach, use this one and let someone elses router find the destination.
As for link-state and hybrid, those types will handle routing updates differently but it all boils down to the same thing. They know about directly connected networks which they can then let other connected routers know about. If "all else fails" they can use a default route and let someone elses router find the destination IP.
Are you asking how each individual routing protocol communicates its routes/changes to other routers?
Question: "how does he acquire this route?(regarding RIP,OSPF,IGRP and EIGRP)"
Routers know about directly connected networks, if you choose to populate those networks in a routing protocol, then you have the possibility of other directly connected routers "learning" about the original routers routes.
In the below example, router 1 and 2 will learn about each others 192.168.x.0 network via RIP. The key is advertising your connected networks to other routers.
You can also configure a static route for traffic that does not match up against a route already in the routing table.
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