diff --git a/ipnet.go b/ipnet.go
index 62b3403..74b5c0b 100644
--- a/ipnet.go
+++ b/ipnet.go
@@ -289,6 +289,7 @@ func NewIPNetwork(ipnet string) (IPNetwork, error) {
 	var mask int
 	index := strings.IndexByte(ipnet, '/')
 	if index == -1 {
+		index = len(ipnet)
 		if strings.IndexByte(ipnet, ':') == -1 {
 			mask = 32
 		} else {
diff --git a/ipnet_test.go b/ipnet_test.go
index 48b6343..ff7ff71 100644
--- a/ipnet_test.go
+++ b/ipnet_test.go
@@ -16,8 +16,21 @@ package netaddr
 
 import (
 	"fmt"
+	"testing"
 )
 
+func TestNewIPNetwork(t *testing.T) {
+	net := MustNewIPNetwork("192.168.10.10/24")
+	if net.String() != "192.168.10.10/24" {
+		t.Error(net.String())
+	}
+
+	net = MustNewIPNetwork("192.168.10.10")
+	if net.String() != "192.168.10.10/32" {
+		t.Error(net.String())
+	}
+}
+
 func ExampleIPNetwork_Network() {
 	net := MustNewIPNetwork("192.168.10.10/24")
 	fmt.Println(net.Network())