diff --git a/Cargo.toml b/Cargo.toml
index faf5334..507816d 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "netdev"
-version = "0.38.2"
+version = "0.39.0"
 authors = ["shellrow <shellrow@foctal.com>"]
 edition = "2024"
 description = "Cross-platform library for network interface"
diff --git a/README.md b/README.md
index 3f2b6da..6bc1489 100644
--- a/README.md
+++ b/README.md
@@ -32,7 +32,7 @@ This project was rebranded from [default-net][default-net-crates-io-url] by the
 Add `netdev` to your dependencies  
 ```toml:Cargo.toml
 [dependencies]
-netdev = "0.38"
+netdev = "0.39"
 ```
 
 For more details, see [examples][examples-url] or [doc][doc-url].  
diff --git a/src/interface/interface.rs b/src/interface/interface.rs
index ac95573..3978093 100644
--- a/src/interface/interface.rs
+++ b/src/interface/interface.rs
@@ -246,3 +246,30 @@ impl Interface {
         crate::stats::counters::update_interface_stats(self)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::interface::interface::Interface;
+    use ipnet::{Ipv4Net, Ipv6Net};
+    use std::net::{IpAddr, Ipv4Addr, Ipv6Addr};
+
+    #[test]
+    fn global_helpers_filter() {
+        let mut itf = Interface::dummy();
+        itf.ipv4 = vec![
+            Ipv4Net::new(Ipv4Addr::new(10, 0, 0, 1), 8).unwrap(), // private
+            Ipv4Net::new(Ipv4Addr::new(1, 1, 1, 1), 32).unwrap(), // global
+        ];
+        itf.ipv6 = vec![
+            Ipv6Net::new(Ipv6Addr::LOCALHOST, 128).unwrap(), // loopback
+            Ipv6Net::new("2606:4700:4700::1111".parse().unwrap(), 128).unwrap(), // global
+        ];
+
+        // Check global_ip_addrs() fillters correctly
+        let globals = itf.global_ip_addrs();
+        assert!(globals.contains(&IpAddr::V4(Ipv4Addr::new(1, 1, 1, 1))));
+        assert!(globals.contains(&IpAddr::V6("2606:4700:4700::1111".parse().unwrap())));
+        assert!(!globals.contains(&IpAddr::V4(Ipv4Addr::new(10, 0, 0, 1))));
+        assert!(!globals.contains(&IpAddr::V6(Ipv6Addr::LOCALHOST)));
+    }
+}
diff --git a/src/interface/mod.rs b/src/interface/mod.rs
index eeeeb87..647e95f 100644
--- a/src/interface/mod.rs
+++ b/src/interface/mod.rs
@@ -93,3 +93,69 @@ pub(crate) fn interfaces() -> Vec<Interface> {
         crate::os::bsd::interface::interfaces()
     }
 }
+
+#[cfg(test)]
+mod tests {
+    #![cfg(feature = "gateway")]
+    use crate::interface::types::InterfaceType;
+    use crate::interface::{interface::Interface, pick_default_iface_index};
+    use ipnet::{Ipv4Net, Ipv6Net};
+    use std::net::{IpAddr, Ipv4Addr, Ipv6Addr};
+
+    #[test]
+    fn exact_match_and_fallback_v4() {
+        let mut a = Interface::dummy();
+        a.index = 1;
+        a.if_type = InterfaceType::Ethernet;
+        a.ipv4 = vec![Ipv4Net::new(Ipv4Addr::new(192, 168, 1, 10), 24).unwrap()];
+
+        let mut b = Interface::dummy();
+        b.index = 2;
+        b.if_type = InterfaceType::Ethernet;
+        b.ipv4 = vec![Ipv4Net::new(Ipv4Addr::new(10, 0, 0, 2), 8).unwrap()];
+
+        // Prefers exact match
+        let local = IpAddr::V4(Ipv4Addr::new(192, 168, 1, 10));
+        assert_eq!(
+            pick_default_iface_index(&[a.clone(), b.clone()], local),
+            Some(1)
+        );
+
+        // Fallback to subnet match
+        let in_subnet = IpAddr::V4(Ipv4Addr::new(10, 0, 5, 23));
+        assert_eq!(pick_default_iface_index(&[a, b], in_subnet), Some(2));
+    }
+
+    #[test]
+    fn exact_match_and_fallback_v6() {
+        let mut a = Interface::dummy();
+        a.index = 11;
+        a.if_type = InterfaceType::Ethernet;
+        a.ipv6 = vec![Ipv6Net::new("2001:db8::10".parse::<Ipv6Addr>().unwrap(), 64).unwrap()];
+
+        let mut b = Interface::dummy();
+        b.index = 22;
+        b.if_type = InterfaceType::Ethernet;
+        b.ipv6 = vec![Ipv6Net::new("2606:4700::2".parse::<Ipv6Addr>().unwrap(), 32).unwrap()];
+
+        // Prefers exact match
+        let local = IpAddr::V6("2001:db8::10".parse().unwrap());
+        assert_eq!(
+            pick_default_iface_index(&[a.clone(), b.clone()], local),
+            Some(11)
+        );
+
+        // Fallback to subnet match
+        let in_subnet = IpAddr::V6("2606:4700::abcd".parse().unwrap());
+        assert_eq!(pick_default_iface_index(&[a, b], in_subnet), Some(22));
+    }
+
+    #[test]
+    fn no_exact_nor_subnet() {
+        let mut a = Interface::dummy();
+        a.index = 3;
+        a.ipv4 = vec![Ipv4Net::new(Ipv4Addr::new(192, 168, 0, 5), 24).unwrap()];
+        let local = IpAddr::V4(Ipv4Addr::new(172, 16, 0, 1));
+        assert_eq!(pick_default_iface_index(&[a], local), None);
+    }
+}
diff --git a/src/net/db/oui.rs b/src/net/db/oui.rs
index 6169d0f..ddf9b6d 100644
--- a/src/net/db/oui.rs
+++ b/src/net/db/oui.rs
@@ -32,3 +32,21 @@ pub fn is_known_loopback_mac(mac: &MacAddr) -> bool {
     let mac = mac.address();
     KNOWN_LOOPBACK_MAC_ADDRESSES.contains(&mac.to_uppercase().as_str())
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::net::db::oui;
+    use crate::net::mac::MacAddr;
+
+    #[test]
+    fn detects_loopback() {
+        let mac: MacAddr = "00:00:00:00:00:00".parse().unwrap();
+        assert!(oui::is_known_loopback_mac(&mac));
+    }
+
+    #[test]
+    fn detects_virtual() {
+        let mac: MacAddr = "00:50:56:AA:BB:CC".parse().unwrap();
+        assert!(oui::is_virtual_mac(&mac));
+    }
+}