How to use
Split cuts a subnet into two equal halves (one more mask bit). The button shows the row's current size, so a /26 row says "Split /26" and becomes two /27 rows. A /32 can't be split.
Join brackets sit to the right of the table. Each bracket covers the rows that came from one parent subnet and is labelled with the parent's mask, for example /23. Press it to merge those rows back into the parent.
Use Columns to show or hide parts of the table. Copy link gives you a link that restores the exact layout. The theme isn't part of the link.
/31 networks are counted as 2 usable hosts (RFC 3021) and /32 as 1 host.