pcap_lookupnet.3pcap man page
pcap_lookupnet — find the IPv4 network number and netmask for a device
#include <pcap/pcap.h> char errbuf[PCAP_ERRBUF_SIZE]; int pcap_lookupnet(const char *device, bpf_u_int32 *netp, bpf_u_int32 *maskp, char *errbuf);
pcap_lookupnet() is used to determine the IPv4 network number and mask associated with the network device device. Both netp and maskp are bpf_u_int32 pointers.
pcap_lookupnet() returns 0 on success and -1 on failure. If -1 is returned, errbuf is filled in with an appropriate error message. errbuf is assumed to be able to hold at least PCAP_ERRBUF_SIZE chars.