This is a Golang script to generate a MySQL query that you can use to block an IP range in Drupal 8. Run it like this: go run blockrange.go 123.456.789 (in a situation where you need to block 123.456.789.*). For Drupal 7, change the table name in row 12 to blocked_ips.

blockrange.go

package main

import (
    "fmt"
    "strconv"
    "os"
)

func main() {

    baseIP := "('" + os.Args[1] + "."
    queryStart := "INSERT INTO ban_ip (ip) VALUES"
    var ipList string
    
    for n := 0; n <= 255; n++ {
        ipList = ipList + baseIP + strconv.Itoa(n) + "')"
        if n == 255 {
            continue;
        }
        ipList = ipList + ", "
    }

    queryEnd := ";"

    fmt.Println(queryStart + ipList + queryEnd)

}

Would be nice to make this interactive (prompt for the baseIP)