Given the IP and netmask, how can I calculate the network address using bash?

Question

In a bash script I have an IP address like 192.168.1.15 and a netmask like 255.255.0.0. I now want to calculate the start address of this network, that means using the &

Answer 1

Some Bash functions summarizing all other answers.

ip2int()
{
    local a b c d
    { IFS=. read a b c d; } <<< $1
    echo $(((((((a << 8) | b) << 8) | c) << 8) | d))
}

int2ip()
{
    local ui32=$1; shift
    local ip n
    for n in 1 2 3 4; do
        ip=$((ui32 & 0xff))${ip:+.}$ip
        ui32=$((ui32 >> 8))
    done
    echo $ip
}

netmask()
# Example: netmask 24 => 255.255.255.0
{
    local mask=$((0xffffffff << (32 - $1))); shift
    int2ip $mask
}


broadcast()
# Example: broadcast 192.0.2.0 24 => 192.0.2.255
{
    local addr=$(ip2int $1); shift
    local mask=$((0xffffffff << (32 -$1))); shift
    int2ip $((addr | ~mask))
}

network()
# Example: network 192.0.2.0 24 => 192.0.2.0
{
    local addr=$(ip2int $1); shift
    local mask=$((0xffffffff << (32 -$1))); shift
    int2ip $((addr & mask))
}