net/interfaces: explicitly check netaddr.IP.Is6 in isUsableV6

Signed-off-by: Matt Layher <mdlayher@gmail.com>

net/interfaces/interfaces.go

@@ -548,7 +548,7 @@ func isUsableV4(ip netaddr.IP) bool {
 // (fc00::/7) are in some environments used with address translation.
 func isUsableV6(ip netaddr.IP) bool {
 	return v6Global1.Contains(ip) ||
-		(ip.IsPrivate() && !tsaddr.TailscaleULARange().Contains(ip))
+		(ip.Is6() && ip.IsPrivate() && !tsaddr.TailscaleULARange().Contains(ip))
 }
 
 var (

net/interfaces/interfaces_test.go

@@ -58,6 +58,8 @@ func TestIsUsableV6(t *testing.T) {
 		{"zeros", "0000:0000:0000:0000:0000:0000:0000:0000", false},
 		{"Link Local", "fe80::1", false},
 		{"Global", "2602::1", true},
+		{"IPv4 public", "192.0.2.1", false},
+		{"IPv4 private", "192.168.1.1", false},
 	}
 
 	for _, test := range tests {