ripv1_mask_host
extern in_addr_t ripv1_mask_host(in_addr_t, const struct interface *);
mask = ripv1_mask_host(dst, ifp);
mask = ripv1_mask_host(dst, ifp);
mask = ripv1_mask_host(dst, ifp);
v1_mask = ripv1_mask_host(htonl(dst_h),