How to Implement Reverse DNS Look Up Cache?

Reverse DNS look up is using an internet IP address to find a domain name. For example, if you type 74.125.200.106 in browser, it automatically redirects to google.in.
How to implement Reverse DNS Look Up cache? Following are the operations needed from cache:

Add an IP address to URL Mapping in cache.
Find URL for a given IP address.

One solution is to use Hashing.
In this post, a Trie based solution is discussed. One advantage of Trie based solutions is, the worst case upper bound is O(1) for Trie, for hashing, the best possible average case time complexity is O(1). Also, with Trie we can implement prefix search (finding all urls for a common prefix of IP addresses).
The general disadvantage of Trie is large amount of memory requirement, this is not a major problem here as the alphabet size is only 11 here. Ten characters are needed for digits from '0' to '9' and one for dot ('.').
The idea is to store IP addresses in Trie nodes and in the last node we store the corresponding domain name. Following is C style implementation in C++.

CPP

// C++ based program to implement reverse DNS lookup
#include<stdio.h>
#include<stdlib.h>
#include<string.h>

// There are atmost 11 different chars in a valid IP address
#define CHARS 11

// Maximum length of a valid IP address
#define MAX 50

// A utility function to find index of child for a given character 'c'
int getIndex(char c) { return (c == '.')? 10: (c - '0'); }

// A utility function to find character for a given child index.
char getCharFromIndex(int i) { return (i== 10)? '.' : ('0' + i); }

// Trie Node.
struct trieNode
{
    bool isLeaf;
    char *URL;
    struct trieNode *child[CHARS];
};

// Function to create a new trie node.
struct trieNode *newTrieNode(void)
{
    struct trieNode *newNode = new trieNode;
    newNode->isLeaf = false;
    newNode->URL = NULL;
    for (int i=0; i<CHARS; i++)
        newNode->child[i] = NULL;
    return newNode;
}

// This method inserts an ip address and the corresponding
// domain name in the trie. The last node in Trie contains the URL.
void insert(struct trieNode *root, char *ipAdd, char *URL)
{
    // Length of the ip address
    int len = strlen(ipAdd);
    struct trieNode *pCrawl = root;

    // Traversing over the length of the ip address.
    for (int level=0; level<len; level++)
    {
        // Get index of child node from current character
        // in ipAdd[].  Index must be from 0 to 10 where
        // 0 to 9 is used for digits and 10 for dot
        int index = getIndex(ipAdd[level]);

        // Create a new child if not exist already
        if (!pCrawl->child[index])
            pCrawl->child[index] = newTrieNode();

        // Move to the child
        pCrawl = pCrawl->child[index];
    }

    //Below needs to be carried out for the last node.
    //Save the corresponding URL of the ip address in the
    //last node of trie.
    pCrawl->isLeaf = true;
    pCrawl->URL = new char[strlen(URL) + 1];
    strcpy(pCrawl->URL, URL);
}

// This function returns URL if given IP address is present in DNS cache.
// Else returns NULL
char  *searchDNSCache(struct trieNode *root, char *ipAdd)
{
    // Root node of trie.