diff --git a/bep40.go b/bep40.go
new file mode 100644
index 00000000..b92ef9d9
--- /dev/null
+++ b/bep40.go
@@ -0,0 +1,82 @@
+package torrent
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"hash/crc32"
+
+	"net"
+)
+
+var table = crc32.MakeTable(crc32.Castagnoli)
+
+type peerPriority = uint32
+
+type ipPort struct {
+	IP   net.IP
+	Port uint16
+}
+
+func sameSubnet(ones, bits int, a, b net.IP) bool {
+	mask := net.CIDRMask(ones, bits)
+	return a.Mask(mask).Equal(b.Mask(mask))
+}
+
+func ipv4Mask(a, b net.IP) net.IPMask {
+	if !sameSubnet(16, 32, a, b) {
+		return net.IPv4Mask(0xff, 0xff, 0x55, 0x55)
+	}
+	if !sameSubnet(24, 32, a, b) {
+		return net.IPv4Mask(0xff, 0xff, 0xff, 0x55)
+	}
+	return net.IPv4Mask(0xff, 0xff, 0xff, 0xff)
+}
+
+func mask(prefix, bytes int) net.IPMask {
+	ret := make(net.IPMask, bytes)
+	for i := range ret {
+		ret[i] = 0x55
+	}
+	for i := 0; i < prefix; i++ {
+		ret[i] = 0xff
+	}
+	return ret
+}
+
+func ipv6Mask(a, b net.IP) net.IPMask {
+	for i := 6; i <= 16; i++ {
+		if !sameSubnet(i*8, 128, a, b) {
+			return mask(i, 16)
+		}
+	}
+	panic(fmt.Sprintf("%s %s", a, b))
+}
+
+func bep40PriorityBytes(a, b ipPort) []byte {
+	if a.IP.Equal(b.IP) {
+		var ret [4]byte
+		binary.BigEndian.PutUint16(ret[0:2], a.Port)
+		binary.BigEndian.PutUint16(ret[2:4], b.Port)
+		return ret[:]
+	}
+	if a4, b4 := a.IP.To4(), b.IP.To4(); a4 != nil && b4 != nil {
+		m := ipv4Mask(a.IP, b.IP)
+		return append(a4.Mask(m), b4.Mask(m)...)
+	}
+	if a6, b6 := a.IP.To16(), b.IP.To16(); a6 != nil && b6 != nil {
+		m := ipv6Mask(a.IP, b.IP)
+		return append(a6.Mask(m), b6.Mask(m)...)
+	}
+	panic(fmt.Sprintf("%s %s", a.IP, b.IP))
+}
+
+func bep40Priority(a, b ipPort) peerPriority {
+	bs := bep40PriorityBytes(a, b)
+	i := len(bs) / 2
+	_a, _b := bs[:i], bs[i:]
+	if bytes.Compare(_a, _b) > 0 {
+		bs = append(_b, _a...)
+	}
+	return crc32.Checksum(bs, table)
+}
diff --git a/bep40_test.go b/bep40_test.go
new file mode 100644
index 00000000..4c82a5ad
--- /dev/null
+++ b/bep40_test.go
@@ -0,0 +1,27 @@
+package torrent
+
+import (
+	"net"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestBep40Priority(t *testing.T) {
+	assert.EqualValues(t, 0xec2d7224, bep40Priority(
+		ipPort{net.ParseIP("123.213.32.10"), 0},
+		ipPort{net.ParseIP("98.76.54.32"), 0},
+	))
+	assert.EqualValues(t, 0xec2d7224, bep40Priority(
+		ipPort{net.ParseIP("98.76.54.32"), 0},
+		ipPort{net.ParseIP("123.213.32.10"), 0},
+	))
+	assert.Equal(t, peerPriority(0x99568189), bep40Priority(
+		ipPort{net.ParseIP("123.213.32.10"), 0},
+		ipPort{net.ParseIP("123.213.32.234"), 0},
+	))
+	assert.EqualValues(t, "\x00\x00\x00\x00", bep40PriorityBytes(
+		ipPort{net.ParseIP("123.213.32.234"), 0},
+		ipPort{net.ParseIP("123.213.32.234"), 0},
+	))
+}