How do you validate an IP address on Android?

To check the IP as it's being entered you might want to use this instead:

private static final Pattern PARTIAl_IP_ADDRESS =
          Pattern.compile("^((25[0-5]|2[0-4][0-9]|[0-1][0-9]{2}|[1-9][0-9]|[0-9])\\.){0,3}"+
                           "((25[0-5]|2[0-4][0-9]|[0-1][0-9]{2}|[1-9][0-9]|[0-9])){0,1}$"); 

ipEditText.addTextChangedListener(new TextWatcher() {                       
    @Override public void onTextChanged(CharSequence s, int start, int before, int count) {}            
    @Override public void beforeTextChanged(CharSequence s,int start,int count,int after) {}            

    private String mPreviousText = "";          
    @Override
    public void afterTextChanged(Editable s) {          
        if(PARTIAl_IP_ADDRESS.matcher(s).matches()) {
            mPreviousText = s.toString();
        } else {
            s.replace(0, s.length(), mPreviousText);
        }
    }
});

API Level 8+:

You can use the Patterns.IP_ADDRESS global regex.

API Level 1-7:

You may directly include this regex in your project if you target devices with android < 2.2:

private static final Pattern IP_ADDRESS
    = Pattern.compile(
        "((25[0-5]|2[0-4][0-9]|[0-1][0-9]{2}|[1-9][0-9]|[1-9])\\.(25[0-5]|2[0-4]"
        + "[0-9]|[0-1][0-9]{2}|[1-9][0-9]|[1-9]|0)\\.(25[0-5]|2[0-4][0-9]|[0-1]"
        + "[0-9]{2}|[1-9][0-9]|[1-9]|0)\\.(25[0-5]|2[0-4][0-9]|[0-1][0-9]{2}"
        + "|[1-9][0-9]|[0-9]))");
Matcher matcher = IP_ADDRESS.matcher("127.0.0.1");
if (matcher.matches()) {
    // ip is correct
}

Patterns.IP_ADDRESS.matcher(url).matches();