diff --git a/src/main.rs b/src/main.rs
index 940b462..ca67821 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -322,9 +322,30 @@ fn expand_path(path: &str) -> PathBuf {
     PathBuf::from(path)
 }
 
-/// Check if a port is available for binding
+/// Check if a port is available for binding.
+///
+/// pg0 connects to the instance over `127.0.0.1`, so the **IPv4 loopback bind
+/// must succeed** for the port to be usable — any IPv4 error (`AddrInUse`, or
+/// `PermissionDenied` for low ports such as 80, …) means unavailable.
+///
+/// PostgreSQL with the default `listen_addresses = 'localhost'` also binds
+/// `::1`, so a port already held by an IPv6 or dual-stack listener — e.g. a
+/// Docker-published `[::]:5432` — must be rejected too; an IPv4-only probe
+/// misses it and silently collides. Only `AddrInUse` from the IPv6 probe is
+/// treated as taken; other IPv6 errors (e.g. IPv6 disabled on the host) are
+/// ignored so they don't make every port look unavailable.
 fn is_port_available(port: u16) -> bool {
-    std::net::TcpListener::bind(("127.0.0.1", port)).is_ok()
+    use std::net::{Ipv4Addr, Ipv6Addr, TcpListener};
+    // pg0 connects over IPv4 loopback — require it to be bindable.
+    if TcpListener::bind((Ipv4Addr::LOCALHOST, port)).is_err() {
+        return false;
+    }
+    // Reject ports held by an IPv6/dual-stack listener; ignore non-AddrInUse
+    // IPv6 errors (e.g. IPv6 disabled).
+    !matches!(
+        TcpListener::bind((Ipv6Addr::LOCALHOST, port)),
+        Err(ref e) if e.kind() == std::io::ErrorKind::AddrInUse
+    )
 }
 
 /// Find an available port, starting from the given port
@@ -1604,3 +1625,41 @@ fn main() {
         process::exit(1);
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::is_port_available;
+    use std::net::{Ipv4Addr, Ipv6Addr, TcpListener};
+
+    /// A port held only on IPv6 loopback must be reported unavailable. The old
+    /// IPv4-only probe returned `true` here, which let pg0 collide with an
+    /// IPv6/dual-stack listener (e.g. Docker's `[::]:5432`).
+    #[test]
+    fn ipv6_held_port_is_unavailable() {
+        let listener = match TcpListener::bind((Ipv6Addr::LOCALHOST, 0)) {
+            Ok(l) => l,
+            Err(_) => return, // IPv6 unavailable on this host — skip.
+        };
+        let port = listener.local_addr().unwrap().port();
+        assert!(!is_port_available(port), "IPv6-held port should be unavailable");
+    }
+
+    /// A port held on IPv4 loopback must be reported unavailable.
+    #[test]
+    fn ipv4_held_port_is_unavailable() {
+        let listener = TcpListener::bind((Ipv4Addr::LOCALHOST, 0)).unwrap();
+        let port = listener.local_addr().unwrap().port();
+        assert!(!is_port_available(port), "IPv4-held port should be unavailable");
+    }
+
+    /// A free port (bound then released) is reported available.
+    #[test]
+    fn free_port_is_available() {
+        let port = TcpListener::bind((Ipv4Addr::LOCALHOST, 0))
+            .unwrap()
+            .local_addr()
+            .unwrap()
+            .port();
+        assert!(is_port_available(port));
+    }
+}