calculating subnet mask? - networking

Question

how can i calculate the subnet mask having ip address 128.2.19.4 and belong to the subnet 128.2.19.0/25.please give me the detail procedure.i want to learn to calculate.

Answer 1

Here's the algorithm with your example:
The subnet mask is just a representation of the "/25" part of your subnet address.
In IPv4, addresses are 32 bits long, the first 25 bits of which are ones:
1111 1111 1111 1111 1111 1111 1000 0000
addresses are given in octets -- 8 bits each:
octet 1 . octet 2 . octet 3 . octet 4
0000 0000 0000 0000 0000 0000 0000 0000
1111 1111 1111 1111 1111 1111 1000 0000
So a decimal representation of each octet is:
255 . 255 . 255 . 128
That means that your subnet mask would be:
255.255.255.128

Answer 2

The subnet mask is a bitmask. 25 means that 25 out of 32 bits (starting from the top) is used for the network, and the rest for the hosts.
In bytes: 128.2.19.0
In binary 10000000 00000010 00010011 00000000
The bitmask: 11111111 11111111 11111111 10000000
Ergo: ------- network ------------ host
The last 7 bits are used for hosts. The bitmask as bytes is 255.255.255.128.

Answer 3

Here's how you can do it in C:
#include <stdio.h>
#include <arpa/inet.h>
uint32_t cidr_to_netmask(uint8_t cidr)
{
uint8_t unset_bits = 32 - cidr;
return ntohl(0xffffffff << unset_bits);
}
int main(void)
{
uint8_t cidr = 25;
uint32_t _netmask = cidr_to_netmask(cidr);
struct in_addr _netmask_addr = { _netmask };
char netmask[16];
if (inet_ntop(AF_INET, (struct in_addr *)&_netmask_addr, (char *)&netmask, sizeof(netmask)) == NULL) {
fprintf(stderr, "error.\n");
return 1;
}
printf("%d = %s\n", cidr, netmask);
return 0;
}
Output:
25 = 255.255.255.128