https://qiita.com/SILKY_MAJOR/items/185825d6bfc2b096eebd#comment-37db4a87773449b70ac5

ip_to_cidr.py

from ipaddress import IPv4Address, IPv4Network, AddressValueError


def read_lines(filename):
    try:
        with open(filename) as f:
            return f.read().splitlines()
    except IOError:
        print(f"Error: {filename}が開けない")
        exit()
    except UnicodeDecodeError:
        print(f"Error: {filename}が読み込めない")
        exit()


def write_lines(filename, lines):
    try:
        with open(filename, "w") as f:
            print(*lines, sep="\n", file=f)
    except IOError:
        print("Error: {filename}が開けない")
        exit()


def lines_to_addrs(lines):
    try:
        return [int(addr)
                for line in lines
                if line
                for addr in IPv4Network(line)]
    except AddressValueError as e:
        print(f"Error : invalid IPv4 address ({e})")
        exit()


def max_subnet_bits(addr):
    for n in range(32):
        if addr & (1 << n):
            return n
    return 32


def addrs_to_cidrs(addrs):
    i, size = 0, len(addrs)
    while i < size:
        bits = max_subnet_bits(addrs[i])
        for bit in range(bits, -1, -1):
            n = 1 << bit
            if i + n <= size and addrs[i + n - 1] == addrs[i] + n - 1:
                yield IPv4Address(addrs[i]), 32 - bit
                i += n
                break


def cidrs_to_lines(cidrs, prefix32):
    return [str(addr) + (f'/{prefix}' if prefix != 32 or prefix32 else '')
            for addr, prefix in cidrs]


def main(in_file, out_file, prefix32=True):
    lines = read_lines(in_file)
    addrs = lines_to_addrs(lines)
    addrs = sorted(set(addrs))
    cidrs = addrs_to_cidrs(addrs)
    lines = cidrs_to_lines(cidrs, prefix32)
    write_lines(out_file, lines)

if __name__ == '__main__':
    main(in_file="ip_list.txt", out_file="out_file.txt")