Question

I need to find the MTU before I can solve the problem but Im not sure how to do it. Can you please help me with explanation thank you.

An IP datagram of Total Length = 4692 Bytes needs to be fragmented by a router so that IP fragments could be sent on Ethernet medium. Assume the default header size of the IP headers in the original IP datagram (and IP fragments) to be 20 Bytes.

After fragmentation done by the router, mention how many fragments (fragmented IP packets) will be created.

Answer 1

Answer : As we know that in ethernet frame size is between 46 bytes to 1500 byte.

Given that Ip datagram is of size 4692 bytes and header size is of 20 bytes so we need to have data fragments from network layer of which should not be more than 1500 bytes .

So MTU at router should be of size 1500 - ip header size .

Which will be of 1480 . Fragmentation will be done as follows : Total bytes 4692

Sno	fragments number	fragment size
1	frag 1	1480 bytes
2	frag 2	1480 bytes
3	frag 3	1480 bytes
4	frag 4	252 bytes

1480 + 1480 + 1480 +252 = 4692 bytes

So at ethernet the frames fragments will be as follows :

20|1480. Fragment 1

20|1480. Fragment 2

20| 1480 fragment 3

20|252 fragment 4

Hence we sent 4 fragments from network layer to data link layer that is ethernet with MTU size of 1500 bytes