在GO中从Ipv6 CIDR获取IPv6地址的数量

Question

Is there anyway to retrieve the maximum numbers of IPv6 from a CIDR range? Currently I have this code:

package main

import (
    "fmt"
    "log"
    "net"
)

func main() {
    ip, ipnet, err := net.ParseCIDR("2001:200:905::/49")
    var ips []string
    if err != nil {
        log.Fatal(err)
    }
    for ip := ip.Mask(ipnet.Mask); ipnet.Contains(ip); inc(ip) {
        ips = append(ips, ip.String())
    }
    fmt.Println(len(ips))
}

func inc(ip net.IP) {
    for j := len(ip)-1; j>=0; j-- {
        ip[j]++
        if ip[j] > 0 {
            break
        }
    }
}

But this process runs so slow. Is there any efficient way of retrieving the total number of ip addresses?