diff --git a/README.md b/README.md
index a9539a3..0569a01 100644
--- a/README.md
+++ b/README.md
@@ -57,4 +57,4 @@ The supported methods are `softnet.policy.get` and `softnet.policy.set`. A compl
 
 Every request must include a non-null string (at most 256 bytes) or non-negative integer `id`; notifications are rejected so policy changes always have an acknowledgment. Policy updates are atomic: all targets are parsed and a new prefix map is built before the active policy changes. Longest-prefix matching and block precedence for identical prefixes are preserved. Targets are normalized and deduplicated, so retrying the active revision with the same policy is idempotent; reusing it with a different policy or retrying a superseded revision returns a JSON-RPC conflict error. A policy may contain at most 4096 combined allow/block targets, and a request frame may not exceed 1 MiB.
 
-`--allow=0.0.0.0/0` additionally disables vmnet bridge isolation during interface creation. Bridge isolation cannot be changed for a running VM, so policy updates that would toggle it are rejected without changing the active policy. Use `block=["0.0.0.0/0"]` with specific allow targets for a default-deny policy. Closing the control socket leaves the last accepted policy active.
+An allow target that normalizes to `0.0.0.0/0` additionally disables vmnet bridge isolation during interface creation. Bridge isolation cannot be changed for a running VM, so policy updates that would toggle it are rejected without changing the active policy. Use `block=["0.0.0.0/0"]` with specific allow targets for a default-deny policy. Closing the control socket leaves the last accepted policy active.
diff --git a/lib/proxy/control.rs b/lib/proxy/control.rs
index 2307f85..4b6e79b 100644
--- a/lib/proxy/control.rs
+++ b/lib/proxy/control.rs
@@ -8,7 +8,7 @@ use jsonrpsee_types::{
         METHOD_NOT_FOUND_CODE as METHOD_NOT_FOUND, PARSE_ERROR_CODE as PARSE_ERROR,
     },
 };
-use prefix_trie::{Prefix, PrefixMap};
+use prefix_trie::PrefixMap;
 use serde::Deserialize;
 use serde_json::{Value, json};
 use smoltcp::wire::Ipv4Address;
@@ -47,9 +47,9 @@ struct PolicyUpdate {
 
 impl Policy {
     pub(super) fn new(gateway_ip: Ipv4Address, allow: Vec<Target>, block: Vec<Target>) -> Self {
-        let bridge_isolation = !allow.contains(&Target::Prefix(Ipv4Net::zero()));
         let allow = normalize_targets(allow);
         let block = normalize_targets(block);
+        let bridge_isolation = Self::bridge_isolation(&allow);
         let rules = build_rules(gateway_ip, &allow, &block);
 
         Policy {
@@ -85,7 +85,7 @@ impl Policy {
 
         let allow = parse_targets(allow)?;
         let block = parse_targets(block)?;
-        let bridge_isolation = !allow.contains(&Target::Prefix(Ipv4Net::zero()));
+        let bridge_isolation = Self::bridge_isolation(&allow);
         let rules = build_rules(self.gateway_ip, &allow, &block);
 
         if self.desired_revision.as_deref() == Some(desired_revision.as_str()) {
@@ -141,6 +141,12 @@ impl Policy {
             self.bridge_isolation,
         )
     }
+
+    pub(super) fn bridge_isolation(allow: &[Target]) -> bool {
+        !allow
+            .iter()
+            .any(|target| matches!(target, Target::Prefix(prefix) if prefix.prefix_len() == 0))
+    }
 }
 
 impl PolicyUpdate {
@@ -818,6 +824,31 @@ mod tests {
         assert_eq!(policy.result(), before);
     }
 
+    #[test]
+    fn noncanonical_default_route_disables_isolation_and_round_trips() {
+        let raw_default = Target::Prefix(Ipv4Net::from_str("10.1.2.3/0").unwrap());
+        assert!(!Policy::bridge_isolation(&[raw_default]));
+
+        let mut policy = policy(&["10.1.2.3/0"], &[]);
+        let initial = policy.result();
+        assert_eq!(initial["allow"], json!(["0.0.0.0/0"]));
+        assert_eq!(initial["bridgeIsolation"], false);
+
+        let response = request(
+            &mut policy,
+            json!({
+                "jsonrpc": "2.0",
+                "id": 1,
+                "method": "softnet.policy.set",
+                "params": {"allow": ["0.0.0.0/0"], "block": [], "desiredRevision": "vm-uid:42"}
+            }),
+        );
+
+        assert_eq!(response["result"]["allow"], json!(["0.0.0.0/0"]));
+        assert_eq!(response["result"]["bridgeIsolation"], false);
+        assert_eq!(response["result"]["desiredRevision"], "vm-uid:42");
+    }
+
     #[test]
     fn validates_json_rpc_envelope_method_and_parameters() {
         let mut policy = policy(&[], &[]);
diff --git a/lib/proxy/mod.rs b/lib/proxy/mod.rs
index fdddf17..fb1d2ef 100644
--- a/lib/proxy/mod.rs
+++ b/lib/proxy/mod.rs
@@ -16,7 +16,6 @@ pub use exposed_port::ExposedPort;
 use ipnet::Ipv4Net;
 use mac_address::MacAddress;
 use port_forwarder::PortForwarder;
-use prefix_trie::Prefix;
 use smoltcp::wire::EthernetFrame;
 use std::io::ErrorKind;
 use std::os::unix::io::{AsRawFd, RawFd};
@@ -71,10 +70,7 @@ impl Proxy<'_> {
         control_fd: Option<RawFd>,
     ) -> Result<Proxy<'proxy>> {
         let vm = VM::new(vm_fd)?;
-        let host = Host::new(
-            vm_net_type,
-            !allow.contains(&Target::Prefix(Ipv4Net::zero())),
-        )?;
+        let host = Host::new(vm_net_type, Policy::bridge_isolation(&allow))?;
         let poller_timeout = Duration::from_millis(100);
         let policy = Policy::new(host.gateway_ip, allow, block);
         let control = control_fd.map(Control::new).transpose()?;