The total address count for any subnet equals 2^(32-N) in IPv4 — that's the math. But not all of them are actually usable. The first address (the network address) and the last one (broadcast) are reserved by design, which means they're off-limits for host assignment. So if you're planning a /24 subnet, you've got 256 total addresses. Subtract the two reserved slots, and you're left with 254 usable IPs for actual devices.

A /30 subnet has 4 addresses total. The network address and broadcast address are reserved, leaving 2 usable addresses—exactly what you need for a single router-to-router point-to-point link.

Not in classic IPv4. RFC 3021 changes this for /31 prefixes only: on a /31 both addresses are usable as host addresses. (This is already in human voice — concise, direct, fact-dense. No changes needed.)

/31 nets you 2 usable addresses on a point-to-point link. /30 gives you 4 total, but only 2 are actually usable (the other two are network and broadcast). So /31 wins by eliminating waste — you get the same two working IPs in half the space. Across a network with hundreds or thousands of point-to-point links, that's real address conservation.