ntohs

ntohs, short for network-to-host short, is a function or macro used in network programming to convert 16-bit values from network byte order to the host machine’s native byte order. Network byte order is defined as big-endian, meaning the most significant byte is transmitted first. The host machine’s byte order may be little-endian or big-endian; ntohs adapts the value to the host’s order by swapping bytes on little-endian systems and leaving it unchanged on big-endian systems.

On POSIX systems, ntohs is declared in arpa/inet.h, and on Windows it is provided in WinSock2.h. The function takes a 16-bit unsigned value in network byte order and returns the same value expressed in the host’s byte order. Its return type is unsigned short, matching the 16-bit length used by common protocol fields.

ntohs is commonly used when interpreting protocol headers where 16-bit fields are transmitted in network order, such as port numbers in TCP and UDP headers. Before performing comparisons or arithmetic on these values within a program, they are converted to host order with ntohs. Example: unsigned short net_port = ...; unsigned short port = ntohs(net_port);

Related functions include htons (host-to-network short) for converting from host order to network order, and ntohl / htonl for 32-bit values. Implementations may be macros or inline functions and may optimize away the conversion on big-endian hosts.