Using sed to search and replace an ip address in a file

If your version of sed supports extended regular expressions (the -r option), you could do something like this (which is similar to what you have in your grep statement). Also note $newip is outside the single quotes to allow the shell to replace it.

sed -r 's/(\b[0-9]{1,3}\.){3}[0-9]{1,3}\b'/"$newip"/

BTW this solution still matches strings that do not represent IP addresses. See this site under IP Adresses for more complex solutions.

sed -e 's/old ip/new ip/g' filename