What is the purpose of IP sets?

A.
They group together IP addresses that are assigned to the same network interfaces.

B.
They group together IP addresses and networks that can be referenced by the network routing table.

C.
They group together IP addresses that can be referenced by netfilter rules.

D.
They group together IP and MAC addresses used by the neighbors on the local network.

E.
They group together IP addresses and user names that can be referenced from /etc/hosts.allow and /etc/hosts.deny

Explanation:
http://ipset.netfilter.org/