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.