diff --git a/peers/manager.go b/peers/manager.go
index 71c0507..20ec25d 100644
--- a/peers/manager.go
+++ b/peers/manager.go
@@ -85,6 +85,25 @@ const (
 	routeSwitchCooldown = 30 * time.Second
 )
 
+// normalizeServerRouteDestination converts the bare IPv4 address returned by
+// Pangolin's site configuration into the host CIDR expected by Newt's route
+// helpers. Pangolin 1.21 returns values such as "100.90.128.4", while the
+// Darwin route and NetworkSettings implementations require "100.90.128.4/32".
+// Preserve already-CIDR values and invalid values so the downstream helper can
+// report its normal validation error.
+func normalizeServerRouteDestination(serverIP string) string {
+	if strings.Contains(serverIP, "/") {
+		return serverIP
+	}
+
+	ip := net.ParseIP(serverIP)
+	if ip == nil || ip.To4() == nil {
+		return serverIP
+	}
+
+	return ip.To4().String() + "/32"
+}
+
 // NewPeerManager creates a new PeerManager with an internal PeerMonitor
 func NewPeerManager(config PeerManagerConfig) *PeerManager {
 	pm := &PeerManager{
@@ -221,7 +240,8 @@ func (pm *PeerManager) AddPeer(siteConfig SiteConfig) error {
 		return err
 	}
 
-	if err := network.AddRouteForServerIPWithSource(siteConfig.ServerIP, pm.interfaceName, pm.localIP); err != nil {
+	serverRouteDestination := normalizeServerRouteDestination(siteConfig.ServerIP)
+	if err := network.AddRouteForServerIPWithSource(serverRouteDestination, pm.interfaceName, pm.localIP); err != nil {
 		logger.Error("Failed to add route for server IP: %v", err)
 	}
 	if err := network.AddRoutesWithSource(siteConfig.RemoteSubnets, pm.interfaceName, pm.localIP); err != nil {
@@ -285,7 +305,8 @@ func (pm *PeerManager) RemovePeer(siteId int) error {
 		return err
 	}
 
-	if err := network.RemoveRouteForServerIPWithSource(peer.ServerIP, pm.interfaceName, pm.localIP); err != nil {
+	serverRouteDestination := normalizeServerRouteDestination(peer.ServerIP)
+	if err := network.RemoveRouteForServerIPWithSource(serverRouteDestination, pm.interfaceName, pm.localIP); err != nil {
 		logger.Error("Failed to remove route for server IP: %v", err)
 	}
 
diff --git a/peers/route_test.go b/peers/route_test.go
new file mode 100644
index 0000000..546cdc4
--- /dev/null
+++ b/peers/route_test.go
@@ -0,0 +1,45 @@
+package peers
+
+import "testing"
+
+func TestNormalizeServerRouteDestination(t *testing.T) {
+	tests := []struct {
+		name     string
+		serverIP string
+		want     string
+	}{
+		{
+			name:     "bare IPv4 address",
+			serverIP: "100.90.128.4",
+			want:     "100.90.128.4/32",
+		},
+		{
+			name:     "IPv4 host CIDR",
+			serverIP: "100.90.128.4/32",
+			want:     "100.90.128.4/32",
+		},
+		{
+			name:     "other IPv4 CIDR",
+			serverIP: "192.0.2.1/24",
+			want:     "192.0.2.1/24",
+		},
+		{
+			name:     "IPv6 address is preserved",
+			serverIP: "2001:db8::4",
+			want:     "2001:db8::4",
+		},
+		{
+			name:     "invalid address is preserved",
+			serverIP: "not-an-ip",
+			want:     "not-an-ip",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := normalizeServerRouteDestination(tt.serverIP); got != tt.want {
+				t.Fatalf("normalizeServerRouteDestination(%q) = %q, want %q", tt.serverIP, got, tt.want)
+			}
+		})
+	}
+}