rings: RingThreePrefix()

Ring 3 corresponds to the pods of the kubernetes cluster of a region

Signed-off-by: Alejandro Mery <amery@jpi.io>

pkg/rings/three.go

@@ -0,0 +1,18 @@
+package rings
+
+import "net/netip"
+
+// RingThreePrefix returns the subnet corresponding to
+// the pods of a cluster.
+//
+// Ring 3 is a `10.(region_id << 4).0.0/12` network
+func RingThreePrefix(region RegionID) (subnet netip.Prefix, err error) {
+	switch {
+	case !region.Valid():
+		err = ErrOutOfRange(region, "region")
+	default:
+		addr := AddrFrom4(10, uint(region)<<4, 0, 0)
+		subnet = netip.PrefixFrom(addr, RingThreeBits)
+	}
+	return subnet, err
+}