Andrew S. Tanenbaum Edition 5th Exercise 5 Question 24 (Page No. 492)

Question

A router is blasting out IP packets whose total length (data plus header) is 1024 bytes.
Assuming that packets live for 10 sec, what is the maximum line speed the router can
operate at without danger of cycling through the IP datagram ID number space?

Answer 1

The IP datagram ID number space is 2^16 = 65535
Packet lifetime = 10 seconds
Therefore, a maximum of 65535 packets may be sent in 10 seconds. If any more packets were to be sent within the 10 seconds, there would be multiple live packets with the same ID.
floor(65535/10) = 6553 packets/sec

The maximum line speed @ 1024 bytes/packet is:
1024 bytes/packet * 6553 packets/sec * 8 bits/byte = 53,682,176 bps (~ 53.7 Mbps) 

The maximum line speed @ 40 bytes/packet is:
40 bytes/packet * 6553 packets/sec * 8 bits/byte = 2,096,960 bps (~2.1 Mbps)