Question

6. (If you cannot get the exact regular expression, put one that is as close as you can. Specify what is wrong with it.)An IP address is often written as four decimal numbers ranging from 0 to 255 with dots between them. For example the database server that we use for this class is 163.238.35.165

a. Write a regular expression that matches IP addresses of this type. You can probably find this answer online, but try to do it yourself first.

b. Often the first two numbers correspond to the network (163.238 in this example) and the second two correspond to the host (35.165 in this example). Write the php code that checks a string to see if it is a valid ip address. If it is, the code prints out the network and host. If it is not, the code prints “ILLEGAL IP ADDRESS”

Write the php code that checks a string to see if it is a valid ip address. If it is, the code prints out the network and host. If it is not, the code prints “ILLEGAL IP ADDRESS”

Answer 1

a) REGEX FOR IP ADDRESS : Please follow image attached. I have mentioned how each octet is represented by arrows and explanation.

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b) PHP CODE TO VALIDATE AN IP ADDRESS

<?php

$ipAddr1 = "163.238.35.165"; // to check for valid ip address
$ipAddr2 = "16.238"; // to check for illegal ip address

/* The filter_var() function filters a variable with the specified filter.
It takes two arguements, first the variable to be filtered and next is
   the FILTER_VALIDATE_IP filter which validates an IP address. */

if (filter_var($ipAddr1, FILTER_VALIDATE_IP))
{
echo("LEGAL IP ADDRESS:: ");
  
/* explode is a funtion to split the a string object into and array of objects
and the splitting is based on a particulat string, like colon, semicolon, dot, /, etc */
  
$ipAddr1Array = explode (".", $ipAddr1);
  
/* after exploding the array looks like this :
ipAddr1Array = { "163", "238", "35", "165"} and we can access by their index starting from 0
length-1.
*/
  
echo " NetworkID : ". $ipAddr1Array[0].".". $ipAddr1Array[1];
echo " and HostID : ". $ipAddr1Array[2].".". $ipAddr1Array[3];
}
else
{
echo("ILLEGAL IP ADDRESS");
}
  
// checking for ipAddr2 which is an illegal address
if (filter_var($ipAddr2, FILTER_VALIDATE_IP))
{
echo("\nLEGAL IP ADDRESS");
}
else
{
echo("\nILLEGAL IP ADDRESS");
}
  
?>