azure: Pick the last IP in the subnet

Picking the last IP in the subnet for the internal LBIP.
Previously, the 5th IP was being picked due to reservations
for the first few IPs but if the users reserver more of the IPs
at the start, the LBIP logic will not work. Choosing the last
IP now to reduce the chances for error.

Author: rna-afk

pkg/asset/manifests/azure/cluster.go

@@ -331,12 +331,14 @@ func getIPWithinCIDR(subnets []*net.IPNet, ip string) string {
 			return ip
 		}
 	}
-	ipSubnets := make(net.IP, len(subnets[0].IP))
-	copy(ipSubnets, subnets[0].IP)
-	// Since the first 4 IP of the subnets are usually reserved[1], pick the next one that's available in the CIDR.
-	// [1] - https://learn.microsoft.com/en-us/azure/virtual-network/ip-services/private-ip-addresses#allocation-method
-	ipSubnets[len(ipSubnets)-1] += 4
-	return ipSubnets.String()
+
+	// Pick the last IP address in the first subnet range using the mask.
+	subnet := *subnets[0]
+	lastIP := make(net.IP, len(subnet.Mask))
+	for i := range subnet.IP {
+		lastIP[i] = subnet.IP[i] | (subnet.Mask[i] ^ 0xFF)
+	}
+	return lastIP.String()
 }
 
 func getNextAvailableIPForLoadBalancer(ctx context.Context, installConfig *installconfig.InstallConfig, lbip string) (string, error) {