Title: htons() / ntohs() / htonl() / ntohl()

From htons(3):

"htons() converts a 16-bit quantity from host byte order to network byte order."

Different CPU architectures group multiple bytes differently. For example, on a "little-endian" machine (an example of which is the Intel CPU), the value 0x1234 is stored in memory as 0x3412. However, on a "big-endian" machine, the value 0x1234 is stored in memory as 0x1234.

It is important that values in a header are sent across a network in a consistent manner independent of the architecture of the sending or receiving system. For this reason, a standard was chosen. The standard chosen was big-endian although it could have just as well been little-endian.

htons() is defined in /include/net/hton.h, as:
#define htons(x) (_tmp=(x), ((_tmp>>8) & 0xff) | ((_tmp<<8) & 0xff00))

ntohs() converts a 16-bit quantity from network byte order to host byte order, the reverse of htons().

htonl() and ntohl() are identical to htons() and ntohs() except that they convert 32-bit quantities instead of 16-bit quantities.

Processes generally supply header information when sending packets. The data in these fields is converted to the network format (i.e., big-endian) by the process before the process copies the data to the network service.