This makes it correct for IPv6 too.

Change-Id: Ide2fc12ed97974b6abbafa66c4b83d0086c4e87c
Reviewed-on: https://go-review.googlesource.com/15030Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>