router has just received the following new IP addresses: $57\mathrm{.}6\mathrm{.}96\mathrm{.}0/21,$

$57\mathrm{.}6\mathrm{.}104\mathrm{.}0/21,57\mathrm{.}6\mathrm{.}112\mathrm{.}0/21,$ and $57\mathrm{.}6\mathrm{.}120\mathrm{.}0/21.$ If all of them use the same outgoing

line, can they be aggregated? If so$,$ to what? If not, why not?