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1. Routing Concept

1. Routing Concept

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Routing Concept – Transcript:

Routing Concept What Will You Learn


Routing Concept

Starting the routing track it is wise to answer the question regarding why do we need routing in ip networks and what exactly is routing? When devices conected to network need to talk to each other they start sending the packets, but depending on the IP network or subnet dependency they behave differently. Let me explain this.

Communication Rule

For the sake of the routing concpept exlanation we can devide the network scenarios into two that force to behave the devices differently. First is where both computers a and b are located within the same ip subnet. The second is where device a belongs to one ip network and seocnd deviced belongs to diffrerent ip network. What is the difference here? In general when sending device is about to send the packet it verifies the ip address of the receiver under the ip locationcondition. Meaning if receiver is in my ip subnet or not.  In the earlier case sender sees that receiver IP is located within the same subnet. So device attached to Ethernet segment ARPs for the physical address that has assigned the receiver’s IP. This behawior is known to us alraedy from the Network services course. ARP is sent with broadcast destination first in layer two segment waiting for replay. After it gets to the receiver, receiver responds with arp reply that he’s mac is the one. Reply goes with unicast back to sender. Then sender updates the arp table and starting from now begins sending packets encapsulating tchem with destiantion mac address of receing device.



The later case is where receiver’s ip is with different subnet than sender thus sender is not able to arp because the arp being broadcast does not go outside the Ethernet segmenet. Single Subnet are most often related to single layer 2 segment (sometimes called vlan) in terms of the boundary. So when transmitting out of subnet scope sender always needs to have default Gateway and this is key word here. Default Gateway is the device or more precisely interface addressed with one of IP from the particular subnet and used for forwarding packets outside this subnet. Each device working within subnet uses default Gateway to reach outside world in terms of ip addressing. So going back to our case, when devices A sends the packet to device B needs to point packet towards default gataway. But it is extremely important to not do the common mistake explaining network. Packet sent towards destination has IP of desintation not IP of default Gateway. Sender needs only to have default gatwey MAC address to send the frame correctly. Then default Gateway decapsulates the frame and route the packet towards destination. This how gateway idea works!


Router Has Routing Information


Now lets imagine that gatway is the interface belonging to particular network device. This device naturaly has more than one interfaces to reach the other networks. Such device takes the packet , looks for destination IP in the packet, find right outgoing interfaces and send the packet towards destination. This proces is called exactly routing and such devices that route the packets are called routers.

Routing Table Check

To properly route the packet router need some information to consult with which internaface to send the packet. This kind of information is called routing table. The routing table Is a piece of memory storing infomration about the detination networks and and how to achieve tchem. To send the packet to desination router also needs to have next devices which is next closest and the local interfaces to send the packet with. The next device is called next hop device.

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