htons(), htonl(), inet_pton() — network byte order

Lesson

Different CPUs store multi-byte integers in different orders ("endianness"). To make networking portable, every multi-byte field in a network packet is stored in network byte order, which is big-endian (most significant byte first).

You convert with:

• htons(x) — host to network short — for 16-bit values like a port.
• htonl(x) — host to network long — for 32-bit values like an IPv4 address.
• ntohs, ntohl — reverse direction.
• inet_pton(family, "127.0.0.1", &addr) — parse a dotted-decimal string into an in_addr. Modern, preferred over the deprecated inet_addr.

Code examples

#include <arpa/inet.h>
#include <stdio.h>

int main(void) {
    /* port 8080 in network byte order */
    unsigned short p = htons(8080);
    printf("port bytes: %02x %02x\n", ((unsigned char*)&p)[0], ((unsigned char*)&p)[1]);

    struct in_addr a;
    inet_pton(AF_INET, "127.0.0.1", &a);
    printf("addr bytes: %08x (network order)\n", a.s_addr);
    return 0;
}

Common mistakes

• Storing a port in host byte order. sin_port = 8080; is wrong; you need sin_port = htons(8080);.
• Using inet_addr (returns INADDR_NONE ambiguously when the string IS 255.255.255.255). Use inet_pton instead.

Summary

Network byte order is big-endian. Use htons/htonl for fields you write into a network packet. Use inet_pton to parse IPs.

Practice with these exercises

• Open a listening TCP socket on localhost