Checking the network with bash

Some bash coding
Ok, I posted this somewhere, but I can't remember where :)
I made a bash script for scanning a sub-network to know which computers are up. I know that nping and anothers programs do this, but I wanted to do it by myself using a shell script, and this is how I did it in the first place:

#!/usr/bin/env bash

PING="$(which ping) -c 1 -W 1"

for((i=1;i<255;i++)); do
    ${PING} ${1}.${i}
    if [ $? -eq 0 ]; then
        echo -e "${1}.${i} is up"
    fi
done

When I ran that script, it worked fine, but very slowly... so I modified it to be this way:

#!/usr/bin/env bash

function pinging(){
    SUBNET=${1}
    LAST=${2}
    PING="$(which ping) -c 1 -W 1"
    ${PING} ${SUBNET}.${LAST} > /dev/null 2>&1
    if [ $? -eq 0 ]; then
        echo -e "${SUBNET}.${LAST} is up"
    fi
}

if [ -z ${2} ]; then
    for((x=1;x<255;x++)); do
        ${0} ${1} ${x} &
    done
else
    pinging ${1} ${2}
fi

I hope this will be useful for someone to use it as learning material (this is not the right thing to do because there are applications availble for this)