Which utility is a line-based interface for managing IP sets in the Linux kernel?

The utility designed for managing IP sets in the Linux kernel is ipset. This command-line tool allows users to create, modify, and delete sets of IP addresses or networks that can be used for firewall rules, specifically with iptables or nftables. By organizing IP addresses into sets, it simplifies the process of managing multiple addresses and improves the performance of firewall rules, as sets can be referenced instead of enumerating individual IP addresses.

In contrast, the other utilities mentioned serve different functions within Linux. Vmstat is used for reporting virtual memory statistics, providing insights into memory usage, processes, and system performance. Mkswap is a utility for setting up swap area on a device or file, which is essential for managing memory in environments with limited RAM. Free displays the amount of free and used memory in the system, helping users monitor their memory resources but does not manage IP sets.

This understanding of ipset's functionality clarifies why it is the correct answer in the context of managing IP addresses within the Linux kernel.