determining whether a valid IP shell

Analysis: IP consist of four digits, in the division point, a set of the most up to 3 and each field not be greater than 255 [.]
# / Bin / the bash!
Function checkip () {

if echo $IP|egrep"^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$" >/dev/null; then
    if [ $VALID_CHECK == "yes" ]; then
     echo "IP $IP  available!"
        return 0
    else
        echo "IP $IP not available!"
        return 1
    fi
else
    echo "IP format error!"
    return 1
fi

}
while true; do
read -p "Please enter IP: " IP
checkip $IP
[ $? -eq 0 ] && break
done