Program for IP forwarding table lookup

In Unix operating system, there is a routing table which contains a certain number of tuples. These tuples are consist of network IP, subnet mask, gateway IP and interface name. These details are used to forward a packet to the outside of its network to connect to the internet. So this article gives an idea of how the system takes the decision when a packet is needed to be forwarded.


Output: eth9
Here, there is no network IP entry in the routing table
which starts with "201". In this case it will 
choose default path(,,, eth9). 
Still, it will perform bitwise AND operation with 
each entry and then chooses default path's interface 
and gateway to send packet outside. Default path means 
interface to which system is directly connected.

Output: eth0
Here bitwise AND operation is performed 
with each entry of routing table and correspondent 
network's interface name and gateway IP is returned.

How forwarding of packet works?

This can be understood easily with the help of a small example:

  • Suppose there is a packet with IP address “” and the routing table has following entries:
    Network’s IP Address Subnet Mask Gateway’s IP Address Interface Name eth4 eth4 eth9 eth1 eth2 eth3
  • So when packet goes to the kernel of the system to find gateway and interface, it will first perform bitwise AND operation with subnet mask of each entry to find Longest prefix match.
  • The result of the bitwise AND operation is then compared with the network’s IP address. So it will return the correspondent IP address of gateway and interface’s name through which packet can go out.
  • Binary representation of is 00010100.10000001.00000000.00000001. It then performs bitwise AND operation with subnet mask for each and every entry of routing table.
  • In this table that entry number is 4( i.e.,,, eth1) which gives the longest prefix match for this packet. So packet will go out from eth1 interface and chooses the gateway for forwarding.

Below is the implementation of the above approach using Linked List data structure. It takes 2 files as an input and returns an output in another file mentioned above.

Input files and Output files

  • File “input.txt” represents the IP address of a packet.
  • File “routing.txt” contains routing table entries to which IP address is going to be matched.
  • File “output.txt” contains outputs for each input.






// C code to implement IP forwarding table lookup
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define M 15
#define N 150
// Declaration of structure of linked list
// to store ip address in each node
struct node {
    char* data;
    struct node* next;
} * head[15];
// This function fetch data from file
// and store them into different arrays
void storeData(FILE* fp,
               char buf[M][N],
               char net[M][N],
               char mask[M][N],
               char gateway[M][N],
               char port[M][N])
    char line[200];
    int c, i = 0, j, k = 0, m = 0;
    // Read data from the file line by line
    // and each line is stored in array separately.
    while (fgets(line, sizeof(line), fp)) {
        j = 0;
        for (int l = 0; l < strlen(line); l++) {
            buf[i][j] = line[l];
    // From each lines stored in buf,
    // network id, subnet mask, gateway
    // and port are extracted
    // and stored into individual arrays.
    for (i = 0; i < 15; i++) {
        k = 0;
        for (j = 0; buf[i][j] != ','; j++) {
            net[i][k] = buf[i][j];
        m = j + 2;
        k = 0;
        for (j = m; buf[i][j] != ','; j++) {
            mask[i][k] = buf[i][j];
        m = j + 2;
        k = 0;
        for (j = m; buf[i][j] != ','; j++) {
            gateway[i][k] = buf[i][j];
        m = j + 2;
        k = 0;
        for (j = m; buf[i][j] != '\0'; j++) {
            port[i][k] = buf[i][j];
// Function to create routing table
// using arrays created by storeData() function
// using linked list data structure
void insert(char net[M][N], char mask[M][N],
            char gateway[M][N], char port[M][N],
            char buf[M][N])
    char *temp1, *temp2, *temp3, *temp4;
    struct node* new;
    for (int i = 0; i < M; i++) {
        // Initialize head of each
        // linked list with NULL.
        head[i] = NULL;
    for (int i = 0; i < M; i++) {
        for (int j = 0; j < 4; j++) {
            // If head is null
            // then first create new node
            // and store network id into it.
            if (head[i] == NULL) {
                new = (struct node*)malloc(
                    sizeof(struct node));
                new->data = net[i];
                new->next = NULL;
                head[i] = new;
            // If head is not null
            // and value of j is 1 then create new node
            // which is pointed by head and it
            // will contain subnet mask
            else if (j == 1) {
                new->next = (struct node*)malloc(
                    sizeof(struct node));
                new = new->next;
                new->data = mask[i];
                new->next = NULL;
            // If head is not null and value of j is 2
            // then create new node
            // which is pointed by subnet mask
            // and it will contain gateway
            else if (j == 2) {
                new->next = (struct node*)malloc(
                    sizeof(struct node));
                new = new->next;
                new->data = gateway[i];
            // If head is not null and value of j is 3
            // then create new node
            // which is pointed by gateway and
            // it will contain port
            else if (j == 3) {
                new->next = (struct node*)malloc(
                    sizeof(struct node));
                new = new->next;
                new->data = port[i];
    // Perform sorting on the basis
    // of longest prefix of subnet mask
    for (int i = 0; i < M; i++) {
        for (int j = i; j < M; j++) {
            // Longest prefix has been compared
            // by using inet_addr() system call
            // which gives decimal value of an ip address.
            if (inet_addr(head[i]->next->data)
                < inet_addr(head[j]->next->data)) {
                struct node* temp = head[i];
                head[i] = head[j];
                head[j] = temp;
// This function will search for gateway ip
// and port number in routing table
// through which packet has been sent
// to next node/destination
void search(FILE* fp1, FILE* fp2)
    char str[100];
    struct in_addr addr;
    unsigned int val;
    fprintf(fp2, "%c", ' ');
    // Read file 'input.txt' line by line
    // and perform bitwise AND between subnet mask
    // and input(destination) ip coming from file.
    while (fgets(str, sizeof(str), fp1)) {
        for (int i = 0; i < M; i++) {
            // Perform bitwise AND operation on result
            // (i.e. Decimal value of an ip address)
            // coming from inet_addr() system call
            val = inet_addr(str) & inet_addr(head[i]->next->data);
            addr.s_addr = val;
            char* str1 = inet_ntoa(addr);
            char* str2 = head[i]->data;
            int count = 0;
            // Compare the network id string with result
            // coming after performing AND operation
            // and if they are same then increment count.
            for (int i = 0; str1[i] != '\0'; i++) {
                if (str1[i] == str2[i]) {
            // If count is same as the string length
            // of network id then find gateway ip
            // and port number of that respective network id
            // and write it into 'output.txt' file.
            if (count == strlen(str1)) {
                struct node* ptr = head[i]->next;
                struct node* temp = ptr->next;
                while (temp != NULL) {
                    fprintf(fp2, "%s ", temp->data);
                    temp = temp->next;
// Driver code
int main(int argc, char* argv[])
    FILE *fin, *fout, *fp;
    char buf[M][N] = { { 0 } };
    char net[M][N] = { { 0 } };
    char mask[M][N] = { { 0 } };
    char gateway[M][N] = { { 0 } };
    char port[M][N] = { { 0 } };
    // if command line argument is less than 3
    // then it will show standard error.
    if (argc < 3) {
        fprintf(stderr, "File name:%s\n", argv[0]);
        return 1;
    // If 3 arguments are given then input
    // and routing.txt files will be opened in read mode
    // while output.txt file is opened in write mode.
    else {
        fin = fopen(argv[1], "r");
        fout = fopen(argv[2], "w");
        fp = fopen(argv[3], "r");
    // If any of the file is not present
    // then it will give an error.
    if (fp == NULL || fin == NULL || fout == NULL) {
        return 0;
    // This function will read the data
    // of a file 'routing.txt' line by line
    // and store them into one array named 'buf',
    // after that the coma separated values in buf
    // are stored into their respective array.
    storeData(fp, buf, net, mask, gateway, port);
    // It will create routing table using linked list
    insert(net, mask, gateway, port, buf);
    // It will take input from input.txt files
    // which contains only destination ip address
    // and search about the route through which
    // packet has been sent in network
    // and output is stored in to an output.txt file
    search(fin, fout);
    printf("Forwarding table has been implemented successfully");
    printf("See the output in %s file\n", argv[2]);
    /*Closes all the files*/
    return 0;
// This code is written by Pooja Patel


Attention reader! Don’t stop learning now. Get hold of all the important CS Theory concepts for SDE interviews with the CS Theory Course at a student-friendly price and become industry ready.

My Personal Notes arrow_drop_up

Check out this Author's contributed articles.

If you like GeeksforGeeks and would like to contribute, you can also write an article using or mail your article to See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.