An ISP is given a block of addresses beginning with $251\mathrm{.}162\mathrm{.}0\mathrm{.}0/16.$ The ISP needs to

distribute these addresses to $3$ groups of customers as follows:

a$)$ Group $1$ has $64$ customers each need $256$ addresses.

b$)$ Group $2$ has $128$ customers each need $128$ addresses.

c$)$ Group $3$ has $128$ customers each need $64$ addresses.

Design the sub$-$blocks and give the slash notation for each sub$-$block. How many

addresses are still available after these allocations?