Something

#!/bin/sh
clear
printf "IP Address?: "
read ip

ipchk=`ifconfig 2>&1 | grep inet | grep -v inet6 | awk '{print $2}' | grep "$ip"`

while [ "$ipchk" != "$ip" ]
  do
        echo "Invalid Ip"
        echo " "
        printf "IP Address?: "
        read ip
        ipchk=`ifconfig 2>&1 | grep inet | grep -v inet6 | awk '{print $2}' | grep "$ip"`
  done

if [ "$ipchk" == "$ip" ] ; then

        ip="$ipchk"

        echo "IP: $ip"
fi


## Results ##

# get-test
IP Address?: l
Invalid Ip

IP Address?: l
Invalid Ip

IP Address?: 145
Invalid Ip

IP Address?: 208.74.173.145
[: 208.74.173.145: unexpected operator