Question

Assume a device with IP address 210.240.57.161/24 wants to send an IP packet to another device, and it only knows that the destination IP address is 210.241.57.40. Explain the steps needed to send the message.

Answer 1

Answer:  If we want to send an IP packet from IP address 210.240.57.161/24 to destination IP address is210.241.57.40 , then first of all we should know that whether  destination IP address is present in the same network or differnet network. Here we will understand the packet flow, in both cases.

Case: 1 If destination IP address is present in the same network (Steps 1-5 are common in both cases)

If we want to  transfer an IP packet from source to destination, both MAC address and IP address of the destination should be known to us. If the destination MAC address is not there then ARP(Address Resolution Protocol) will help us in solving  the issue first then the packet will be delivered to destination host.

Here are the steps to be performed:

1) First of all the source ARP cache is check is performed to check whether the ARP is resolved or not. If the ARP is not resolved, it puts the packet on hold and generates an ARP request.

==> If the ARP is already resolved i.e if we know the destination MAC address, then the packet will be delivered to destination host.

2)If ARP is not resolved , then the ARP request is broadcast all over the network to find out the device having destination IP address.

3) If the device having the destination IP address receives that ARP request send by source ARP cache, it updates it’s own ARP cache.

4)Then destination ARP cache generates reply containing it’s own MAC address.

5) Now we are having the source and destination IP and MAC addresses Now an AND operation is performed between source IP address, source subnet mask and destination IP address, source subnet mask. If the output of both is similar then the destination is present in the same network as source network otherwise it is present in different network.

6) Now, the ARP request will be broadcast first for the target IP address within the network because routers do not forward broadcast packet.The broadcast request is recieved by the switch.

7) So,the switch will broadcast the ARP request and make entry in the ethernet header .

8) Now the ICMP akcnowledgement packet will be unicast from destination to host i.e. destination  can successfully exhange information from host .

Note: The reason why  switch can unicast the reply because the switch has put an entry for source  in its MAC table when the source  broadcast the ARP request.in the same way.Switch has also put an entry for the destination when the switch recieves the ARP reply.

Case-2: If destination IP address is present in the different network

==> Here also the first five steps are same which will help us in getting the MAC address of the destination.

Here are the next steps to be performed:

1) First of all using AND operation we will come to know that destination IP address is present in the different network.

2) Now,the packet is delivered to the default gateway first which in turn delivers the packet to the destination IP address.Here, ICMP packet will be unicast to the default gateway considering that ARP is resolved now.

3) In this way the communication will be setup between source and destination.

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