Remaining IPs in a range/CIDR - go

Question

I would like to find the remaining IPs in a range (CIDR).
For example:
prefixLen, bits := n.Mask.Size()
return 1 << (uint64(bits) - uint64(prefixLen)), nil
This return the # of IPs in the range where n is *net.IPNet.
But say if I have few addresses that are already given out and want to know how many are left 'starting' from some IP addresses, how can I do it?

Answer 1

Here's something you could do
package main
import (
"fmt"
"net"
)
func inet_aton(ip string) uint32 {
var rv uint32
b := net.ParseIP(ip).To4()
rv=uint32(b[0]) << 24
rv=rv+uint32(b[1]) << 16
rv=rv+uint32(b[2]) << 8
rv=rv+uint32(b[3])
return rv
}
func main() {
start := inet_aton("192.168.0.1") //192.168.2.0/20 start
fin := inet_aton("192.168.15.255") //ditto end
here := inet_aton("192.168.15.248") //current allocation
fmt.Printf("you have %d remaining\n",(fin-start) - (here-start))
}

Answer 2

Here is a way to do it that works for both IPv4 and IPv6 using the IPAddress Go library. Disclaimer: I am the project manager.
func remaining(cidr string) (used, remaining, total *big.Int) {
cidrStr := ipaddr.NewIPAddressString(cidr)
cidrBlock := cidrStr.GetAddress().ToPrefixBlock()
host := cidrStr.GetHostAddress()
used = new(big.Int).Sub(host.GetValue(), cidrBlock.GetValue())
remaining = new(big.Int).Sub(cidrBlock.GetUpperValue(), host.GetValue())
total = cidrBlock.GetCount()
fmt.Printf("\nHost %s is address number %d of %d addresses in block %s.\n"+
"There remains %d addresses.\n", host, used, total, cidrBlock,
remaining)
return
}
If you want to stick with IPv4, then you can replace big.Int with unsigned int types.
func remainingIPv4(cidr string) (used, remaining uint32, total uint64) {
cidrStr := ipaddr.NewIPAddressString(cidr)
cidrBlock := cidrStr.GetAddress().ToPrefixBlock().ToIPv4()
host := cidrStr.GetHostAddress().ToIPv4()
used = host.Uint32Value() - cidrBlock.Uint32Value()
remaining = cidrBlock.UpperUint32Value() - host.Uint32Value()
total = cidrBlock.GetIPv4Count()
fmt.Printf("\nHost %s is address number %d of %d addresses in block %s.\n"+
"There remains %d addresses.\n", host, used, total, cidrBlock,
remaining)
return
}
Example:
package main
import (
"fmt"
"github.com/seancfoley/ipaddress-go/ipaddr"
"math/big"
)
func main() {
remaining("192.168.2.0/20")
remaining("2001:0db8:85a3::8a2e:0370:7334/64")
remainingIPv4("192.168.2.0/20")
}
Example output:
Host 192.168.2.0 is address number 512 of 4096 addresses in block 192.168.0.0/20.
There remains 3583 addresses.
Host 2001:db8:85a3::8a2e:370:7334 is address number 151930230829876 of 18446744073709551616 addresses in block 2001:db8:85a3::/64.
There remains 18446592143478721739 addresses.
Host 192.168.2.0 is address number 512 of 4096 addresses in block 192.168.0.0/20.
There remains 3583 addresses.