Split, size, summarise — IPv4 and IPv6.
One requirement per line, as name hosts or just a number. Largest are placed first and each block is aligned to its own size.
One network per line. Overlaps, duplicates and unsorted input are fine; an address with host bits set is read as its network.
Take the lowest network address and the highest broadcast address. The summary is the longest prefix they have in common; every bit after that differs somewhere in the range.
Exact means the summary holds exactly the inputs and nothing else. If it is not exact, advertising it attracts traffic for the extra space — use the minimal list instead, which is the fewest blocks that cover the inputs and nothing more.
A block must start on a multiple of its own size. Place a small one first and the next big one has to skip ahead to the next boundary, stranding the space between. Largest first keeps every later block aligned for free.
A /64 is one LAN. An ISP typically hands a site a /48 (65,536 LANs) or /56 (256). The first 64 bits route; the last 64 are the host. The mask table for IPv4 is on the subnet page.