Why does Ethernet have a minimum frame size of 64 bytes?

The IEEE 802.3 states that there can be a maximum of five physical segments between any two nodes, with four repeaters between the nodes. When using 10Base5 cabling, each physical segment can be up to 500 meters long. Therefore, an Ethernet network’s maximum linear length is 2500 meters.


Speed of light in copper is 2/3 of the speed of light in vacuum.


In order to detect collision an adapter must transmit for at least 25 microseconds.


At 10Mbps 
After mth collision, choose K randomly from {0, …, 2m-1}
and wait for K*512 bit times before trying again
For 10Mbps = K * 51.2 microseconds


So 


12 bytes  - Src+Dst MAC address
2 bytes - Ethernet type
4 bytes - CRC
----------
18 bytes
----------


Preamble = 8 bytes, 7 bytes of 10101010 + 1 byte of Start of frame delimiter 10101011
IFG - 12 bytes
Jamming signal - 4 bytes

_- 

This leaves 46 bytes as the packet length


Now IP + TCP header = 40 bytes. Therefore, the OS must pad the data before sending it to the adapter.