A router has just received the following new IP addresses: 88.62.108.0/21, 88.62.116.0/21, 88.62.120.0/21 and 88.62.128.0/21. If all of them use the same outgoing interface, can they be aggregated? If so, to what? If not, why not?

Yes, they can be aggregated into a single network address of 88.62.0.0/16

Explanation:

Since they all will be using the same outgoing interface so they can be grouped according to next hop which appears to be the same for all four IP address.

First convert all the network addresses into binary.

DECIMAL                      BINARY                                                      CLASS OF IP

88.62.108.0/21            01011000.00111110.01101100.00000000      Class A

88.62.116.0/21             01011000.00111110.01110100.00000000     Class A

88.62.120.0/21            01011000.00111110.01111000.00000000     Class A

88.62.128.0/21           01011000.00111110.10000000.00000000    class A

Looking at the binary form of the address from left to right it can be observed that all for address are identical in the first two octet, that is up to the 16th digit.

So the new subnet mask will be /16, using any of the four address and the /16 mask will can obtain the new network address

All four address can be summarized as 88.62.0.0/16