To avoid flow and congestion control problems in the ARPANET, a host or IMP would send a packet and wait for a RFNM $($Request for Next Message, pronounced $$rough$-$numb$).$ The ARPANET had $56$Kbps lines and a maximum PDU size of $1024$ bits. Assuming that the speed of light in copper is $2/3$rds the speed of light in a vacuum $($c $=3$ x $10^5$ km$/$sec$),$ was this a reasonable design? When would it no longer be a reasonable design?given a company $6$ departments each having $10$ hosts what mask could be applied to divide the up the network equally