Steps taken by a Router on receiving a packet.

Question

A Router outside the organization received a packet 224.240.7.91. Consider following steps that a router may take: 
1. The router looks in its routing table to find out how to route the packet to the destination 
2. The router looks at the first byte of the address to find the class 
3. The default mask for the class is ANDED (logical AND operation) and address is found.

Which of the above steps actually the router takes?

In case, more than one steps are taken, mention the sequence of steps also.

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According to me, a Router takes the following steps:

a) If the ethernet destination address of the arriving frame belongs to the router, accept the frame, else drop it.

b) Examine the IP version and length of the datagram.

c) Decrement the TTL, update the IP header checksum.

d) Check to see if TTL == 0.

e) If the IP destination address is in the forwarding table, forward to the correct outgoing port(s) for the next hop.

f) Find the ethernet destination address for the next hop router.

g) Create a new ethernet frame and send it. 

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There should not be any need of finding the class of the IP address if we are going to use forwarding table for finding the next hop.So step 2 should not be taken.

Also if the router will have an entry for the destination IP address of the packet, it will forward the packet otherwise it will simply drop the packet.So step 3 should not be taken.

So the router should take only step 1.

Am I missing something?

What should be the correct answer to this question?

Comments

I think the order should be 3 2 1.
All the steps must be taken.

Answer 1

u went too deep . what i think this happens in short,

take the first entry of the rounting table and and with subnet mask of that entry, if it is match just send it to the interface, else go to next . till you hit a default entry, i think router does not use the class address instead it already have the n/w id + subnet mask of n/w.

so according to me only one true step is 1.

Comments

I agree. ANurag went too deep. Step 3 is part of step 1. So, only step 1 should be correct.

Answer 2

See that thing is your answer has 2 cases

Case 1: If the addressing is classful addressing

if this is the case then the router looks for first two bytes. This is necessary because it detremines the class and hence it’s corresponding mask. So the step is requisite.

Case 2: If the addressing is classless addressing

If this is the case then you actually have the mask along with the IP and so there is no need to check for the bits.