net: add example for CIDRMask

I had trouble translating the documentation language into a subnet
- e.g. whether /31 was CIDRMask(1, 31) or CIDRMask(1, 32) or
CIDRMask(31, 32) so I thought I'd add a short example showing how to
create the right masks.

Change-Id: Ia6a6de08c5c30b6d2249b3194cced2d3c383e317
Reviewed-on: https://go-review.googlesource.com/32677Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/example_test.go

@@ -5,6 +5,7 @@
 package net_test
 
 import (
+	"fmt"
 	"io"
 	"log"
 	"net"
@@ -34,3 +35,13 @@ func ExampleListener() {
 		}(conn)
 	}
 }
+
+func ExampleCIDRMask() {
+	// This mask corresponds to a /31 subnet
+	fmt.Println(net.CIDRMask(31, 32))
+
+	// This mask corresponds to a /64 subnet for a IPv6 mask
+	fmt.Println(net.CIDRMask(64, 128))
+	// Output: fffffffe
+	// ffffffffffffffff0000000000000000
+}