Ports < IPPORT_RESERVED are reserved for privileged processes (e.g. root). Ports > IPPORT_USERRESERVED are reserved for servers, not necessarily privileged.
Definitions of bits in internet address integers. On subnets, the decomposition of addresses to host and net parts is done according to subnet mask, not the masks here.
Define a macro to stuff the loopback address into an Internet address 0x0100007F is htonl(INADDR_LOOPBACK) without the overhead of a function call.
Options for use with [gs]etsockopt at the IP level. NB. The negative option values below are not supported and must not be used in calls to the [gs]etsockopt() API as they will be ignored or cause an error.