Sockets - Byte Order

There are two types of byte ordering: big endian and little endian

Byte Ordering

Integer types (char, short, int, long) can be represented in memory as either big endian or little endian
Assuming memory addresses increase from left to right, big endian is how you'd write the actual bytes -- with low order bytes in higher memory location
Little endian is just the opposite, low order bytes would be stored in lower memory locations

Which end of a multi-byte value (big or little) is stored in the lower memory address


unsigned short = 1;


                M E M O R Y   A D D R E S S
                ---------------------------->
                0x00                  0x01
                +------------------+------------------+
big endian      | high order byte  | low order byte   |
                |                  |                  |
                | 0 0 0 0 0 0 0 0  | 0 0 0 0 0 0 0 1  |
                +------------------+------------------+
                
                +------------------+------------------+
little endian   | low order byte   | high order byte  |
                |                  |                  |
                | 0 0 0 0 0 0 0 1  | 0 0 0 0 0 0 0 0  |
                +------------------+------------------+

Host Byte Order vs. Network Byte Order

host byte order - the byte order used by a given system. I.e. Intel x86 processors use little endian.
network byte order - the byte order used by a network protocol. I.e. IP uses big endian.

When writing to socket address structures, port #'s (16 bits) and IP addresses (32 bits), MUST be converted to network byte order

#include <netinet/in.h>

/*
 * HOST => NETWORK
 */

/* unsigned 16 bit */
uint16_t htons (uint16_t 16bithostvalue);

/* unsigned 32 bit */
uint32_t htonl (uint32_t 32bithostvalue);


/*
 * NETWORK => HOST
 */

/* unsigned 16 bit */
uint16_t ntohs (uint16_t 16bitnetworkvalue);

/* unsigned 32 bit */
uint32_t ntohl (uint32_t 32bitnetworkvalue);