diff --git a/chord/id.go b/chord/id.go
new file mode 100644
index 0000000..59480c2
--- /dev/null
+++ b/chord/id.go
@@ -0,0 +1,44 @@
+package chord
+
+import (
+	"bytes"
+	"crypto/sha1"
+	"net/netip"
+)
+
+type ID [20]byte
+
+func addrID(addr netip.AddrPort) ID {
+	if !addr.IsValid() {
+		return ID{}
+	}
+	b := make([]byte, 0, len("[ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff%enp5s0]:65535"))
+	b = addr.AppendTo(b)
+	return keyID(b)
+}
+
+func keyID[T string | []byte](key T) ID {
+	s := sha1.Sum([]byte(key))
+	return s
+}
+
+// contains reports whether a clockwise traversal through the hash space
+// starting immediately after left reaches x no later than it reaches right.
+func contains(left, right, x ID) bool {
+	lr := bytes.Compare(left[:], right[:])
+	lx := bytes.Compare(left[:], x[:])
+	xr := bytes.Compare(x[:], right[:])
+	switch {
+	case lr < 0:
+		// left is less than right, so the interval does not cross the origin.
+		// |      L-----R      |
+		return (lx < 0) == (xr <= 0)
+	case lr > 0:
+		// The interval crosses the origin.
+		// |-----R      L-----|
+		return (lx < 0) != (xr <= 0)
+	default:
+		// The interval is the entire hash space.
+		return true
+	}
+}
diff --git a/chord/id_test.go b/chord/id_test.go
new file mode 100644
index 0000000..c55019c
--- /dev/null
+++ b/chord/id_test.go
@@ -0,0 +1,30 @@
+package chord
+
+import "testing"
+
+func TestContains(t *testing.T) {
+	true := [][3]ID{
+		{{1}, {1}, {0}},
+		{{1}, {1}, {1}},
+		{{1}, {1}, {2}},
+		{{1}, {2}, {2}},
+		{{1}, {3}, {2}},
+		{{3}, {1}, {1}},
+		{{3}, {1}, {0}},
+	}
+	false := [][3]ID{
+		{{1}, {2}, {0}},
+		{{1}, {2}, {3}},
+		{{3}, {1}, {2}},
+	}
+	for _, c := range true {
+		if !contains(c[0], c[1], c[2]) {
+			t.Errorf("%02x not found inside %02x - %02x", c[2], c[0], c[1])
+		}
+	}
+	for _, c := range false {
+		if contains(c[0], c[1], c[2]) {
+			t.Errorf("%02x found inside %02x - %02x", c[2], c[0], c[1])
+		}
+	}
+}