Four types of network-related information

Information	Data file	Structure	Keyed lookup functions
Hosts	/etc/hosts	hostent	gethostbyaddr, gethostbyname
Networks	/etc/networks	netent	getnetbyaddr,getnetbyname
Protocols	/etc/protocols	protoent	getprotobyname,getprotobynumber
Services	/etc/services	servent	getservbyname,getservbyport