Code Examples 3: Subnetting and Other Subnet Operations
Sean C Foley edited this page Oct 6, 2023
·
48 revisions
- Iterate through Subnet
- Iterate through Subnet Boundaries
- From Start and End Address, Get a Minimal List of CIDR Blocks Spanning the Range
- From Start and End Address, Get a Single CIDR Block Covering Both
- Merge a List of Addresses or Subnets into a Minimal List of CIDR Blocks
- Break a CIDR Prefix Block into Direct Component Blocks
- Iteratively Break a CIDR Prefix Block into Component Blocks
- Derive New Subnet from Existing CIDR Subnet
- Remove Address or Subnet from Subnet
- Variable Length Subnetting
- Automatic Subnetting
- From Start and End Address, Get a Minimal List of Spanning Subnets, each with Segments either Single-Valued or Full-Range
Use with caution, subnets can get quite large, an IPv6 /64 subnet has 18,446,744,073,709,551,616 addresses.
The basic structure is this:
subnet := ipaddr.NewIPAddressString("192.168.1.0/24").GetAddress().WithoutPrefixLen()
iterator := subnet.Iterator()
for next := iterator.Next(); next != nil; next = iterator.Next() {
fmt.Println(next)
}The call to WithoutPrefixLen is optional, it removes the prefix length from the subnet and all iterated addresses. The first 3 of the 256 lines of output:
192.168.1.0
192.168.1.1
192.168.1.2
Here we iterate through all, but print only beginning and end.
show("192.168.5.0/24")
func show(subnetStr string) {
iterateAll(ipaddr.NewIPAddressString(subnetStr).GetAddress(), 3)
}
func iterateAll(subnet *ipaddr.IPAddress, edgeCount int) {
count := subnet.GetCount()
bigEdge := new(big.Int).SetInt64(int64(edgeCount))
currCount := new(big.Int).Set(count)
i := 0
iterator := subnet.Iterator()
for iterator.HasNext() {
addr := iterator.Next()
currCount.Sub(currCount, big.NewInt(1))
if i < edgeCount { // initial values
i++
fmt.Printf("%d: %v\n", i, addr)
} else if currCount.Cmp(bigEdge) < 0 { // end values
var countDup big.Int
fmt.Printf("%d: %v\n", countDup.Sub(count, currCount), addr)
} else if i == edgeCount {
fmt.Printf("skipping %v addresses\n",
new(big.Int).Sub(count, big.NewInt(2*int64(edgeCount))))
i++
}
}
}Output:
1: 192.168.5.0/24
2: 192.168.5.1/24
3: 192.168.5.2/24
skipping 250 addresses
254: 192.168.5.253/24
255: 192.168.5.254/24
256: 192.168.5.255/24
You can use ToPrefixBlock to get the containing block, if not a zero-host subnet.
subnet := ipaddr.NewIPAddressString("1.186.0.1/15").GetAddress().
ToPrefixBlock().WithoutPrefixLen()
iterator := subnet.Iterator()
listNum := 4
for i := 0; i < listNum && iterator.HasNext(); i++ {
fmt.Println(iterator.Next())
}
count := subnet.GetCount() // count of all addresses in the subnet
fmt.Println(count.Sub(count, big.NewInt(int64(listNum))), "more addresses...")Output:
1.186.0.0
1.186.0.1
1.186.0.2
1.186.0.3
131068 more addresses...
The addresses are iterated in order starting from the zero host, so this variation omits the zero host and max host. In IPv4, those are the network and broadcast addresses, respectively.
subnet := ipaddr.NewIPAddressString("192.168.1.0/29").GetAddress()
if iterator := subnet.Iterator(); iterator.HasNext() {
iterator.Next()
for addr := iterator.Next(); iterator.HasNext(); addr = iterator.Next() {
fmt.Println(addr)
}
}Output:
192.168.1.1/29
192.168.1.2/29
192.168.1.3/29
192.168.1.4/29
192.168.1.5/29
192.168.1.6/29
List addresses at the beginning and end of a subnet's address range. Unlike the previous example, no caution is required with large subnets.
showEdges("2001:db8:abcd:0012::/64")
func showEdges(subnetStr string) {
iterateEdges(ipaddr.NewIPAddressString(subnetStr).GetAddress(), 3)
}
func iterateEdges(subnet *ipaddr.IPAddress, edgeCount int) {
if subnet.GetCount().Cmp(big.NewInt(2*(int64(edgeCount)+1))) < 0 {
// the subnet is small, just print it
i := 1
iterator := subnet.Iterator()
for ; iterator.HasNext(); i++ {
fmt.Printf("%d: %v\n", i, iterator.Next())
}
} else {
// print low boundary addresses
lower := subnet.GetLower()
for increment := 0; increment < edgeCount; increment++ {
fmt.Printf("%d: %v\n",
increment+1, lower.Increment(int64(increment)))
}
count := subnet.GetCount()
fmt.Printf("skipping %v addresses\n",
new(big.Int).Sub(count, big.NewInt(2*int64(edgeCount))))
// print high boundary addresses
upper := subnet.GetUpper()
for decrement := 1 - edgeCount; decrement <= 0; decrement++ {
dec64 := int64(decrement)
dec := big.NewInt(dec64)
fmt.Printf("%d: %v\n", dec.Add(count, dec), upper.Increment(dec64))
}
}
}Output:
1: 2001:db8:abcd:12::/64
2: 2001:db8:abcd:12::1/64
3: 2001:db8:abcd:12::2/64
skipping 18446744073709551610 addresses
18446744073709551614: 2001:db8:abcd:12:ffff:ffff:ffff:fffd/64
18446744073709551615: 2001:db8:abcd:12:ffff:ffff:ffff:fffe/64
18446744073709551616: 2001:db8:abcd:12:ffff:ffff:ffff:ffff/64
toPrefixBlocks("2001:db8:abcd:0012:1::", "2001:db8:abcd:0012:3::")
toPrefixBlocks("1.1.1.111", "1.1.1.120")
func toPrefixBlocks(str1, str2 string) {
string1, string2 := ipaddr.NewIPAddressString(str1), ipaddr.NewIPAddressString(str2)
addr1, addr2 := string1.GetAddress(), string2.GetAddress()
rng := addr1.SpanWithRange(addr2)
blocks := rng.SpanWithPrefixBlocks()
fmt.Printf("Starting with range %v,\nCIDR prefix blocks are %v\n\n", rng, blocks)
}Output:
Starting with range 2001:db8:abcd:12:1:: -> 2001:db8:abcd:12:3::,
CIDR prefix blocks are [2001:db8:abcd:12:1::/80 2001:db8:abcd:12:2::/80 2001:db8:abcd:12:3::/128]
Starting with range 1.1.1.111 -> 1.1.1.120,
CIDR prefix blocks are [1.1.1.111/32 1.1.1.112/29 1.1.1.120/32]
In the example above, the prefix blocks span the range exactly. If you are looking for just a single prefix block including both addresses:
toPrefixBlock("2001:db8:abcd:0012:1::", "2001:db8:abcd:0012:3::")
toPrefixBlock("1.1.1.111", "1.1.1.120")
func toPrefixBlock(str1, str2 string) {
string1, string2 := ipaddr.NewIPAddressString(str1), ipaddr.NewIPAddressString(str2)
addr1, addr2 := string1.GetAddress(), string2.GetAddress()
fmt.Printf("Starting with range %v,\nCIDR prefix blocks are %v\n\n",
addr1.SpanWithRange(addr2), addr1.CoverWithPrefixBlockTo(addr2))
}Output:
Starting with range 2001:db8:abcd:12:1:: -> 2001:db8:abcd:12:3::,
CIDR prefix block is 2001:db8:abcd:12::/78
Starting with range 1.1.1.111 -> 1.1.1.120,
CIDR prefix block is 1.1.1.96/27
print(merge("209.152.214.112/30", "209.152.214.116/31", "209.152.214.118/31"))
print(merge("209.152.214.112/30", "209.152.214.116/32", "209.152.214.118/31"))
print(merge("1:2:3:4:8000::/65", "1:2:3:4::/66", "1:2:3:4:4000::/66",
"1:2:3:5:4000::/66", "1:2:3:5::/66", "1:2:3:5:8000::/65"))
func print(addresses []*ipaddr.IPAddress) {
fmt.Printf("blocks are %v\n", addresses)
}
func merge(strs ...string) []*ipaddr.IPAddress {
first := ipaddr.NewIPAddressString(strs[0]).GetAddress()
remaining := make([]*ipaddr.IPAddress, 0, len(strs)-1)
for _, str := range strs[1:] {
remaining = append(remaining, ipaddr.NewIPAddressString(str).GetAddress())
}
return first.MergeToPrefixBlocks(remaining...)
}Output:
blocks are [209.152.214.112/29]
blocks are [209.152.214.112/30 209.152.214.116/32 209.152.214.118/31]
blocks are [1:2:3:4::/63]
Starting with a prefix block subnet, you can increase the prefix length of the subnet, while not changing the address values themselves. Then the set of addresses have multiple different prefixes. Then you can iterate through those prefix blocks.
adjustBlock("192.168.0.0/28", 2)
func adjustBlock(original string, bitShift int) {
subnet := ipaddr.NewIPAddressString(original).GetAddress()
newSubnets := subnet.SetPrefixLen(subnet.GetPrefixLen().Len() + bitShift)
subnetSet := make([]*ipaddr.IPAddress, 0, newSubnets.GetPrefixCount().Uint64())
iterator := newSubnets.PrefixBlockIterator()
for next := iterator.Next(); next != nil; next = iterator.Next() {
subnetSet = append(subnetSet, next)
}
fmt.Printf("original block: %v\nprefix length increased by %d: %v\nsubnets: %v\n\n",
subnet, bitShift, newSubnets, subnetSet)
}Output:
original block: 192.168.0.0/28
prefix length increased by 2: 192.168.0.0-12/30
subnets: [192.168.0.0/30 192.168.0.4/30 192.168.0.8/30 192.168.0.12/30]
You can iterate through prefixes repeatedly, in different orders.
Iterate, ordered by block size (breadth-first traversal):
subnet := ipaddr.NewIPAddressString("192.168.0.0/29").GetAddress()
listByDecreasingSize(subnet, 1)
func listByDecreasingSize(subnet *ipaddr.IPAddress, bitShift int) {
subnetPrefixLength := subnet.GetPrefixLen()
fmt.Printf("%v size %v\n", subnet, subnet.GetCount())
nextIndent := ""
subPrefLen := subnetPrefixLength.Len()
for subnetPrefixBits := subPrefLen; subnetPrefixBits+bitShift <=
subnet.GetBitCount(); {
// adjust original subnet prefix length
subnetPrefixBits += bitShift
blocks := subnet.SetPrefixLen(subnetPrefixBits)
// iterate through all prefixes
blocksIterator := blocks.PrefixBlockIterator()
for {
next := blocksIterator.Next()
node := fmt.Sprintf("%v size %d", next, next.GetCount())
fmt.Print(nextIndent)
if blocksIterator.HasNext() {
fmt.Print(leftElbow)
fmt.Println(node)
} else {
fmt.Print(rightElbow)
fmt.Println(node)
break
}
}
nextIndent += belowElbows
}
}
const (
leftElbow = "\u251C\u2500" // |-
betweenElbows = "\u2502 " // |
rightElbow = "\u2514\u2500" // '-
belowElbows = " "
)Output:
192.168.0.0/29 size 8
├─192.168.0.0/30 size 4
└─192.168.0.4/30 size 4
├─192.168.0.0/31 size 2
├─192.168.0.2/31 size 2
├─192.168.0.4/31 size 2
└─192.168.0.6/31 size 2
├─192.168.0.0/32 size 1
├─192.168.0.1/32 size 1
├─192.168.0.2/32 size 1
├─192.168.0.3/32 size 1
├─192.168.0.4/32 size 1
├─192.168.0.5/32 size 1
├─192.168.0.6/32 size 1
└─192.168.0.7/32 size 1
Iterate, ordered by block containment (depth-first traversal):
subnet := ipaddr.NewIPAddressString("192.168.0.0/29").GetAddress()
listByContainment(subnet, 1)
func listByContainment(subnet *ipaddr.IPAddress, bitShift int) {
recurseBlocks(subnet, "", "", bitShift)
}
func recurseBlocks(subnet *ipaddr.IPAddress, indent, secondIndent string, bitShift int) {
fmt.Printf("%s%v size %d\n", indent, subnet, subnet.GetCount())
prefLength := subnet.GetPrefixLen()
if prefLength.Len() >= subnet.GetBitCount() {
return
}
subnetPrefixBits := prefLength.Len() + bitShift
blocks := subnet.SetPrefixLen(subnetPrefixBits)
// iterate through those blocks
blocksIterator := blocks.PrefixBlockIterator()
for {
next := blocksIterator.Next()
if blocksIterator.HasNext() {
recurseBlocks(next, secondIndent+leftElbow,
secondIndent+betweenElbows, bitShift)
} else {
recurseBlocks(next, secondIndent+rightElbow,
secondIndent+belowElbows, bitShift)
if subnetPrefixBits >= subnet.GetBitCount() {
fmt.Println(secondIndent)
}
break
}
}
}Output:
192.168.0.0/29 size 8
├─192.168.0.0/30 size 4
│ ├─192.168.0.0/31 size 2
│ │ ├─192.168.0.0/32 size 1
│ │ └─192.168.0.1/32 size 1
│ │
│ └─192.168.0.2/31 size 2
│ ├─192.168.0.2/32 size 1
│ └─192.168.0.3/32 size 1
│
└─192.168.0.4/30 size 4
├─192.168.0.4/31 size 2
│ ├─192.168.0.4/32 size 1
│ └─192.168.0.5/32 size 1
│
└─192.168.0.6/31 size 2
├─192.168.0.6/32 size 1
└─192.168.0.7/32 size 1
The examples above iterate through prefixes, but if you want to derive a specific subnet rather than iterate through the list:
adjustSubnet("192.168.0.0/28", 2, "0.0.0.12")
adjustSubnet("2001:0db8:85a3::/64", 3, "0:0:0:0:e000::")
func adjustSubnet(original string, bitShift int, additionalPrefStr string) {
subnet := ipaddr.NewIPAddressString(original).GetAddress()
fmt.Printf("original block: %v with count %d\n",
subnet, subnet.GetCount())
additionalPrefix := ipaddr.NewIPAddressString(additionalPrefStr).
GetAddress()
newSubnet, _ := subnet.AdjustPrefixLenZeroed(bitShift)
specifiedSubnet, _ := newSubnet.BitwiseOr(additionalPrefix)
fmt.Printf("prefix length increased by %d: %v with count %d\n",
bitShift, newSubnet, newSubnet.GetCount())
fmt.Printf("after inserting additional prefix: %v with count %d\n",
specifiedSubnet, specifiedSubnet.GetCount())
originalSubnet := specifiedSubnet.AdjustPrefixLen(-bitShift).ToPrefixBlock()
fmt.Printf("back to original %v with count %d\n\n",
originalSubnet, originalSubnet.GetCount())
}Output:
original block: 192.168.0.0/28 with count 16
prefix length increased by 2: 192.168.0.0/30 with count 4
after inserting additional prefix: 192.168.0.12/30 with count 4
back to original 192.168.0.0/28 with count 16
original block: 2001:db8:85a3::/64 with count 18446744073709551616
prefix length increased by 3: 2001:db8:85a3::/67 with count 2305843009213693952
after inserting additional prefix: 2001:db8:85a3:0:e000::/67 with count 2305843009213693952
back to original 2001:db8:85a3::/64 with count 18446744073709551616
subnet := "192.0.10.0/28"
exclude(subnet, "192.0.10.1")
exclude(subnet, "192.0.10.2/31")
func exclude(addrStr, sub string) {
one, two := ipaddr.NewIPAddressString(addrStr).GetAddress(),
ipaddr.NewIPAddressString(sub).GetAddress()
result := one.Subtract(two)
fmt.Printf("Removing %v from %v results in: %v\n", two, one, result)
var blockList []*ipaddr.IPAddress
for _, addr := range result {
blockList = append(blockList, addr.SpanWithPrefixBlocks()...)
}
fmt.Printf("converted to prefix blocks: %v\n\n", blockList)
}Output:
Removing 192.0.10.1 from 192.0.10.0/28 results in: [192.0.10.0 192.0.10.2-14/31]
converted to prefix blocks: [192.0.10.0/32 192.0.10.2/31 192.0.10.4/30 192.0.10.8/29]
Removing 192.0.10.2/31 from 192.0.10.0/28 results in: [192.0.10.0/31 192.0.10.4-12/30]
converted to prefix blocks: [192.0.10.0/31 192.0.10.4/30 192.0.10.8/29]
In this example is code that can produce the subnetting shown in some online examples of subnetting, here and here as well as the calculator shown here.
For the quick and easy solution see the next example. This example shows a longer solution that can be customized.
We have a number of groups of hosts, each group to be allocated a subnet. We will represent each host group with the hostgroup type, and each resulting subnet allocation with the allocation type:
type hostGroup struct {
name string
count uint
}
func (group hostGroup) String() string {
return fmt.Sprint(group.name, " of size ", group.count)
}
type allocation struct {
hostGroup
subnet *ipaddr.IPAddress
}
func (alloc allocation) String() string {
return fmt.Sprint(alloc.hostGroup, " from ", alloc.subnet)
}We start with one or more CIDR blocks of adequate size for the subnetting, the availableBlocks list. Throughout, the availableBlocks list will maintain the list of available CIDR prefix block subnets, sorted by block size from smallest to largest. We want the smallest in the list first because it is more efficient to try to allocate from smaller blocks first,
using the smallest block that can fit the number of required hosts.
After we are done, each allocation will have an assigned CIDR block subnet for its host group, and the availableBlocks list will have the unused CIDR blocks that can be used for additional host groups at a future time.
The host groups list is also sorted by size, but instead by largest groups first. We will allocate a subnet for each host group in turn, starting with the largest.
func allocateHosts(availableBlocks []string, toAllocate []hostGroup,
reservedPerBlock uint) {
availableBlockAddrs := make([]*ipaddr.IPAddress, 0, len(availableBlocks))
for _, availableBlock := range availableBlocks {
block := ipaddr.NewIPAddressString(availableBlock).GetAddress()
if block == nil || !block.IsSinglePrefixBlock() {
fmt.Printf("%s is not a valid prefix block subnet\n",
availableBlock)
continue
}
availableBlockAddrs = append(availableBlockAddrs, block)
}
if allocs, unusedBlocks, err :=
allocate(availableBlockAddrs, toAllocate, reservedPerBlock); err != nil {
fmt.Println("unable to allocate:", err)
} else {
fmt.Println("\nallocated subnets:\n", allocs,
"\nremaining unused blocks:\n", unusedBlocks)
}
}
func allocate(
availableBlocks []*ipaddr.IPAddress,
toAllocate []hostGroup,
reservedPerBlock uint) (
allocated []allocation, unused []*ipaddr.IPAddress, err error) {
// sort requests by size, largest first
sort.Slice(toAllocate, func(i, j int) bool {
return toAllocate[i].count > toAllocate[j].count
})
// sort available blocks by size, largest prefix length (smallest block) first
sort.Slice(availableBlocks, func(i, j int) bool {
return availableBlocks[i].GetPrefixLen().Compare(
availableBlocks[j].GetPrefixLen()) > 0
})
allocated = make([]allocation, 0, len(toAllocate))
unused = availableBlocks
for _, group := range toAllocate { // allocate each group
alloc := allocation{hostGroup: group}
if alloc.subnet, unused, err =
assign(unused, group, reservedPerBlock); err != nil {
break
}
allocated = append(allocated, alloc)
}
return
}We iterate through the available blocks until we find one large enough, a subnet whose address count matches or exceeds the required number. Each subnet may have a number of reserved addresses, which for IPv4 may include the network and broadcast addresses, so the subnet size must match or exceed the total of required hosts and reserved addresses. The selected block will be partitioned so that we do not waste address space.
func assign(
availableBlocks []*ipaddr.IPAddress,
group hostGroup,
reserved uint) (
*ipaddr.IPAddress, []*ipaddr.IPAddress, error) {
requiredSize := new(big.Int).SetUint64(uint64(group.count + reserved))
for i, candidate := range availableBlocks {
if candidate.GetCount().Cmp(requiredSize) < 0 {
continue // candidate not big enough
}
// use the candidate for the allocation
allocated, unused := partition(candidate, group, reserved)
// remove the partitioned block from the list
availableBlocks = append(availableBlocks[:i], availableBlocks[i+1:]...)
// insert the unused back into the available list
if unused != nil {
availableBlocks = insertUnused(availableBlocks, unused, i)
}
return allocated, availableBlocks, nil
}
return nil, availableBlocks, fmt.Errorf("no block big enough for %s", group)
}To partition the selected block, we calculate the prefix bit-length for the required block size. Then we iterate through the equal-sized blocks of that prefix length, keeping the first for the host group, and returning the remaining blocks for insertion back into the list of available blocks.
func partition(
block *ipaddr.IPAddress,
group hostGroup,
reserved uint) (
allocated *ipaddr.IPAddress, unused *ipaddr.IPAddressSeqRange) {
sizeRequired := group.count + reserved
bitsRequired := math.Ceil(math.Log2(float64(sizeRequired)))
newPrefixBitCount := block.GetBitCount() - ipaddr.BitCount(bitsRequired)
newPrefixLen := ipaddr.PrefixBitCount(newPrefixBitCount)
if !block.GetPrefixLen().Equal(&newPrefixLen) {
iterator := block.SetPrefixLen(newPrefixBitCount).PrefixBlockIterator()
allocated = iterator.Next()
unused = iterator.Next().GetLower().SpanWithRange(block.GetUpper())
} else {
allocated = block
}
return
}
func insertUnused(
availableBlocks,
unused []*ipaddr.IPAddress,
index int) []*ipaddr.IPAddress {
unusedBlocks := unused.SpanWithPrefixBlocks()
// sort unusedBlocks blocks by size, smallest block first, same as availableBlocks
sort.Slice(unusedBlocks, func(i, j int) bool {
return unusedBlocks[i].GetPrefixLen().Compare(
unusedBlocks[j].GetPrefixLen()) > 0
})
for len(unusedBlocks) > 0 {
lastIndex := len(unusedBlocks) - 1
lastUnused := unusedBlocks[lastIndex]
for ; index > 0; index-- {
if availableBlocks[index-1].GetPrefixLen().Compare(
lastUnused.GetPrefixLen()) >= 0 {
break
}
}
availableBlocks = append(availableBlocks[:index+1], availableBlocks[index:]...)
availableBlocks[index] = lastUnused
unusedBlocks = unusedBlocks[:lastIndex]
}
return availableBlocks
}Now we are ready to run the code with a couple of examples. In the first example from Study-CCNA.com we have 5 host groups (of host group sizes 50, 30, 20, 2, 2, and 2) to assign from the single block 192.168.10.0/24.
var adjustment uint = 2 // reserve the network and broadcast address in each IPv4 subnet
allocateHosts(
[]string{
"192.168.10.0/24",
},
[]hostGroup{
{"HQ LAN", 50},
{"BRANCH 1", 30},
{"BRANCH 2", 20},
{"WAN 1", 2},
{"WAN 2", 2},
{"WAN 3", 2},
},
adjustment)Output:
allocated subnets:
[HQ LAN of size 50 from 192.168.10.0/26 BRANCH 1 of size 30 from 192.168.10.64/27 BRANCH 2 of size 20 from 192.168.10.96/27 WAN 1 of size 2 from 192.168.10.128/30 WAN 2 of size 2 from 192.168.10.132/30 WAN 3 of size 2 from 192.168.10.136/30]
remaining unused blocks:
[192.168.10.140/30 192.168.10.144/30 192.168.10.148/30 192.168.10.152/30 192.168.10.156/30 192.168.10.160/30 192.168.10.164/30 192.168.10.168/30 192.168.10.172/30 192.168.10.176/30 192.168.10.180/30 192.168.10.184/30 192.168.10.188/30 192.168.10.192/26]
In the second example we have 4 host groups (of host group sizes 60, 12, 12, and 28) to assign from the same example block 192.168.10.0/24.
allocateHosts(
[]string{
"192.168.10.0/24",
},
[]hostGroup{
{"Perth", 60},
{"Sydney", 12},
{"Singapore", 12},
{"Kuala Lumpur", 28},
},
adjustment) // reserve the network and broadcast addressesOutput:
allocated subnets:
[Perth of size 60 from 192.168.10.0/26 Kuala Lumpur of size 28 from 192.168.10.64/27 Sydney of size 12 from 192.168.10.96/28 Singapore of size 12 from 192.168.10.112/28]
remaining unused blocks:
[192.168.10.128/26 192.168.10.192/26]
Also note that this code works equally well with IPv6, although with IPv6 the address space is much larger, and thus splitting up the blocks efficiently may result in a large number of unused blocks. Using variable length prefixes for efficiency is typically not as important with IPv6.
The previous example illustrates variable-length subnetting that can be customized as desired.
However, the algorithm outlined in that example is frequently suitable without any customization. If no further customization is required, you can simply use the PrefixBlockAllocator, which follows the same algorithm.
type allocatedBlock = ipaddr.AllocatedBlock[*ipaddr.IPAddress]
allocatedBlocks := alloc([]string{"192.168.10.0/24"}, 30, 20, 2, 2, 2)
for i, allocated := range allocatedBlocks {
fmt.Println(i+1, allocated)
}
func alloc(blockStrs []string, blockSizes ...uint64) []allocatedBlock {
blocks := make([]*ipaddr.IPAddress, len(blockStrs))
for i, str := range blockStrs {
blocks[i] = ipaddr.NewIPAddressString(str).GetAddress()
}
fmt.Println("Allocating subnets from", blocks,
"for blocks of size", blockSizes)
var allocator ipaddr.PrefixBlockAllocator[*ipaddr.IPAddress]
allocator.AddAvailable(blocks...)
allocator.SetReserved(2) // 2 reserved per block
return allocator.AllocateSizes(blockSizes...)
}Output:
Allocating subnets from [192.168.10.0/24] for blocks of size [30 20 2 2 2]
1 192.168.10.0/27 for 30 hosts and 2 reserved addresses
2 192.168.10.32/27 for 20 hosts and 2 reserved addresses
3 192.168.10.64/30 for 2 hosts and 2 reserved addresses
4 192.168.10.68/30 for 2 hosts and 2 reserved addresses
5 192.168.10.72/30 for 2 hosts and 2 reserved addresses
From Start and End Address, Get a Minimal List of Spanning Subnets, each with Segments either Single-Valued or Full-Range
Given an address range, we can first split into a minimal list of sequential blocks. From there, we can split them further so that no segment in the final list is neither single-valued nor full-range. As in many examples, this code works for both IPv4 and IPv6.
func example(lowerStr, upperStr string) {
lower, upper := ipaddr.NewIPAddressString(lowerStr), ipaddr.NewIPAddressString(upperStr)
rng := lower.GetAddress().SpanWithRange(
upper.GetAddress())
// first we split into sequential address blocks
blocks := rng.SpanWithSequentialBlocks()
fmt.Printf(
"Starting with %v, the minimal list of sequential blocks is: %v\n",
rng, blocks)
// now we split further
fmt.Println("Splitting each so all segments" +
" are either single or full range:")
allSplits := splitBlocks(blocks)
fmt.Printf("Total block count: %d\n", len(allSplits))
// we now go in the reverse direction
reverse(rng, allSplits)
}The code to split each block further finds the single segment that is neither single-valued nor full-range. We iterate over the segment values in that segment to produce the subnets for the final list. Earlier examples iterated over prefix blocks, which was iterating over the network bits, the initial part of a subnet. Here we iterate over the initial segments.
func splitBlocks(blocks []*ipaddr.IPAddress) []*ipaddr.IPAddress {
// A sequential block can have at most one segment
// that is not single nor full size,
// so for each block we iterate over that one segment
//ArrayList<IPAddress> allSplits = new ArrayList<IPAddress>();
var allSplits []*ipaddr.IPAddress
for _, block := range blocks {
isSplit := false
if block.IsMultiple() {
for i := 0; i < block.GetSegmentCount(); i++ {
segment := block.GetSegment(i)
if segment.IsMultiple() {
if segment.IsFullRange() {
break
}
// segment is neither single nor full-range
allSplits = split(block, i, allSplits)
isSplit = true
break
}
}
}
if !isSplit {
label := "address"
if block.IsMultiple() {
label = "subnet"
}
fmt.Printf("\tNot splitting %s %v\n", label, block)
allSplits = append(allSplits, block)
}
}
return allSplits
}
func split(block *ipaddr.IPAddress, index int, allSplits []*ipaddr.IPAddress) []*ipaddr.IPAddress {
fmt.Println("\tSplitting", block)
iterator := block.BlockIterator(index + 1)
addrs := make([]*ipaddr.IPAddress, 0, block.GetMaxSegmentValue()+1)
for next := iterator.Next(); next != nil; next = iterator.Next() {
addrs = append(addrs, next)
}
isLastSegment := index == block.GetSegmentCount()-1
label := "subnets"
if isLastSegment {
label = "addresses"
}
fmt.Printf("\t\t%d %s: %v\n", len(addrs), label, addrs)
allSplits = append(allSplits, addrs...)
return allSplits
}This additional code shows how to go in the reverse direction, first merging to a minimal list of blocks, then merging to a minimal list of ranges. Alternatively, the code could be shortened by switching the blocks to ranges right away, following with just a single merge of ranges.
func reverse(rng *ipaddr.IPAddressSeqRange, allSplits []*ipaddr.IPAddress) {
// merge to a minimal list of blocks
merged := rng.GetLower().MergeToSequentialBlocks(allSplits...)
fmt.Printf("Merged back to minimal sequential blocks again:\n%v\n", merged)
// switch to ranges so we can merge to a minimal list of ranges
ranges := make([]*ipaddr.IPAddressSeqRange, 0, len(merged))
for _, addr := range merged {
ranges = append(ranges, addr.ToSequentialRange())
}
fmt.Printf("Merged back to range again:\n%v\n\n", ranges[0].Join(ranges...))
}We show the output for a couple of sample ranges, the first a CIDR prefix block:
example("41.216.124.0", "41.216.127.255")Output:
Starting with 41.216.124.0 -> 41.216.127.255, the minimal list of sequential blocks is: [41.216.124-127.*]
Splitting each so all segments are either single or full range:
Splitting 41.216.124-127.*
4 subnets: [41.216.124.* 41.216.125.* 41.216.126.* 41.216.127.*]
Total block count: 4
Merged back to minimal sequential blocks again:
[41.216.124-127.*]
Merged back to range again:
[41.216.124.0 -> 41.216.127.255]
The second example is not a block, resulting in a much larger list:
example("128.1.0.1", "191.255.255.255")Output:
Starting with 128.1.0.1 -> 191.255.255.255, the minimal list of sequential blocks is: [128.1.0.1-255 128.1.1-255.* 128.2-255.*.* 129-191.*.*.*]
Splitting each so all segments are either single or full range:
Splitting 128.1.0.1-255
255 addresses: [128.1.0.1 128.1.0.2 128.1.0.3 128.1.0.4 128.1.0.5 128.1.0.6 128.1.0.7 128.1.0.8 128.1.0.9 128.1.0.10 128.1.0.11 128.1.0.12 128.1.0.13 128.1.0.14 128.1.0.15 128.1.0.16 128.1.0.17 128.1.0.18 128.1.0.19 128.1.0.20 128.1.0.21 128.1.0.22 128.1.0.23 128.1.0.24 128.1.0.25 128.1.0.26 128.1.0.27 128.1.0.28 128.1.0.29 128.1.0.30 128.1.0.31 128.1.0.32 128.1.0.33 128.1.0.34 128.1.0.35 128.1.0.36 128.1.0.37 128.1.0.38 128.1.0.39 128.1.0.40 128.1.0.41 128.1.0.42 128.1.0.43 128.1.0.44 128.1.0.45 128.1.0.46 128.1.0.47 128.1.0.48 128.1.0.49 128.1.0.50 128.1.0.51 128.1.0.52 128.1.0.53 128.1.0.54 128.1.0.55 128.1.0.56 128.1.0.57 128.1.0.58 128.1.0.59 128.1.0.60 128.1.0.61 128.1.0.62 128.1.0.63 128.1.0.64 128.1.0.65 128.1.0.66 128.1.0.67 128.1.0.68 128.1.0.69 128.1.0.70 128.1.0.71 128.1.0.72 128.1.0.73 128.1.0.74 128.1.0.75 128.1.0.76 128.1.0.77 128.1.0.78 128.1.0.79 128.1.0.80 128.1.0.81 128.1.0.82 128.1.0.83 128.1.0.84 128.1.0.85 128.1.0.86 128.1.0.87 128.1.0.88 128.1.0.89 128.1.0.90 128.1.0.91 128.1.0.92 128.1.0.93 128.1.0.94 128.1.0.95 128.1.0.96 128.1.0.97 128.1.0.98 128.1.0.99 128.1.0.100 128.1.0.101 128.1.0.102 128.1.0.103 128.1.0.104 128.1.0.105 128.1.0.106 128.1.0.107 128.1.0.108 128.1.0.109 128.1.0.110 128.1.0.111 128.1.0.112 128.1.0.113 128.1.0.114 128.1.0.115 128.1.0.116 128.1.0.117 128.1.0.118 128.1.0.119 128.1.0.120 128.1.0.121 128.1.0.122 128.1.0.123 128.1.0.124 128.1.0.125 128.1.0.126 128.1.0.127 128.1.0.128 128.1.0.129 128.1.0.130 128.1.0.131 128.1.0.132 128.1.0.133 128.1.0.134 128.1.0.135 128.1.0.136 128.1.0.137 128.1.0.138 128.1.0.139 128.1.0.140 128.1.0.141 128.1.0.142 128.1.0.143 128.1.0.144 128.1.0.145 128.1.0.146 128.1.0.147 128.1.0.148 128.1.0.149 128.1.0.150 128.1.0.151 128.1.0.152 128.1.0.153 128.1.0.154 128.1.0.155 128.1.0.156 128.1.0.157 128.1.0.158 128.1.0.159 128.1.0.160 128.1.0.161 128.1.0.162 128.1.0.163 128.1.0.164 128.1.0.165 128.1.0.166 128.1.0.167 128.1.0.168 128.1.0.169 128.1.0.170 128.1.0.171 128.1.0.172 128.1.0.173 128.1.0.174 128.1.0.175 128.1.0.176 128.1.0.177 128.1.0.178 128.1.0.179 128.1.0.180 128.1.0.181 128.1.0.182 128.1.0.183 128.1.0.184 128.1.0.185 128.1.0.186 128.1.0.187 128.1.0.188 128.1.0.189 128.1.0.190 128.1.0.191 128.1.0.192 128.1.0.193 128.1.0.194 128.1.0.195 128.1.0.196 128.1.0.197 128.1.0.198 128.1.0.199 128.1.0.200 128.1.0.201 128.1.0.202 128.1.0.203 128.1.0.204 128.1.0.205 128.1.0.206 128.1.0.207 128.1.0.208 128.1.0.209 128.1.0.210 128.1.0.211 128.1.0.212 128.1.0.213 128.1.0.214 128.1.0.215 128.1.0.216 128.1.0.217 128.1.0.218 128.1.0.219 128.1.0.220 128.1.0.221 128.1.0.222 128.1.0.223 128.1.0.224 128.1.0.225 128.1.0.226 128.1.0.227 128.1.0.228 128.1.0.229 128.1.0.230 128.1.0.231 128.1.0.232 128.1.0.233 128.1.0.234 128.1.0.235 128.1.0.236 128.1.0.237 128.1.0.238 128.1.0.239 128.1.0.240 128.1.0.241 128.1.0.242 128.1.0.243 128.1.0.244 128.1.0.245 128.1.0.246 128.1.0.247 128.1.0.248 128.1.0.249 128.1.0.250 128.1.0.251 128.1.0.252 128.1.0.253 128.1.0.254 128.1.0.255]
Splitting 128.1.1-255.*
255 subnets: [128.1.1.* 128.1.2.* 128.1.3.* 128.1.4.* 128.1.5.* 128.1.6.* 128.1.7.* 128.1.8.* 128.1.9.* 128.1.10.* 128.1.11.* 128.1.12.* 128.1.13.* 128.1.14.* 128.1.15.* 128.1.16.* 128.1.17.* 128.1.18.* 128.1.19.* 128.1.20.* 128.1.21.* 128.1.22.* 128.1.23.* 128.1.24.* 128.1.25.* 128.1.26.* 128.1.27.* 128.1.28.* 128.1.29.* 128.1.30.* 128.1.31.* 128.1.32.* 128.1.33.* 128.1.34.* 128.1.35.* 128.1.36.* 128.1.37.* 128.1.38.* 128.1.39.* 128.1.40.* 128.1.41.* 128.1.42.* 128.1.43.* 128.1.44.* 128.1.45.* 128.1.46.* 128.1.47.* 128.1.48.* 128.1.49.* 128.1.50.* 128.1.51.* 128.1.52.* 128.1.53.* 128.1.54.* 128.1.55.* 128.1.56.* 128.1.57.* 128.1.58.* 128.1.59.* 128.1.60.* 128.1.61.* 128.1.62.* 128.1.63.* 128.1.64.* 128.1.65.* 128.1.66.* 128.1.67.* 128.1.68.* 128.1.69.* 128.1.70.* 128.1.71.* 128.1.72.* 128.1.73.* 128.1.74.* 128.1.75.* 128.1.76.* 128.1.77.* 128.1.78.* 128.1.79.* 128.1.80.* 128.1.81.* 128.1.82.* 128.1.83.* 128.1.84.* 128.1.85.* 128.1.86.* 128.1.87.* 128.1.88.* 128.1.89.* 128.1.90.* 128.1.91.* 128.1.92.* 128.1.93.* 128.1.94.* 128.1.95.* 128.1.96.* 128.1.97.* 128.1.98.* 128.1.99.* 128.1.100.* 128.1.101.* 128.1.102.* 128.1.103.* 128.1.104.* 128.1.105.* 128.1.106.* 128.1.107.* 128.1.108.* 128.1.109.* 128.1.110.* 128.1.111.* 128.1.112.* 128.1.113.* 128.1.114.* 128.1.115.* 128.1.116.* 128.1.117.* 128.1.118.* 128.1.119.* 128.1.120.* 128.1.121.* 128.1.122.* 128.1.123.* 128.1.124.* 128.1.125.* 128.1.126.* 128.1.127.* 128.1.128.* 128.1.129.* 128.1.130.* 128.1.131.* 128.1.132.* 128.1.133.* 128.1.134.* 128.1.135.* 128.1.136.* 128.1.137.* 128.1.138.* 128.1.139.* 128.1.140.* 128.1.141.* 128.1.142.* 128.1.143.* 128.1.144.* 128.1.145.* 128.1.146.* 128.1.147.* 128.1.148.* 128.1.149.* 128.1.150.* 128.1.151.* 128.1.152.* 128.1.153.* 128.1.154.* 128.1.155.* 128.1.156.* 128.1.157.* 128.1.158.* 128.1.159.* 128.1.160.* 128.1.161.* 128.1.162.* 128.1.163.* 128.1.164.* 128.1.165.* 128.1.166.* 128.1.167.* 128.1.168.* 128.1.169.* 128.1.170.* 128.1.171.* 128.1.172.* 128.1.173.* 128.1.174.* 128.1.175.* 128.1.176.* 128.1.177.* 128.1.178.* 128.1.179.* 128.1.180.* 128.1.181.* 128.1.182.* 128.1.183.* 128.1.184.* 128.1.185.* 128.1.186.* 128.1.187.* 128.1.188.* 128.1.189.* 128.1.190.* 128.1.191.* 128.1.192.* 128.1.193.* 128.1.194.* 128.1.195.* 128.1.196.* 128.1.197.* 128.1.198.* 128.1.199.* 128.1.200.* 128.1.201.* 128.1.202.* 128.1.203.* 128.1.204.* 128.1.205.* 128.1.206.* 128.1.207.* 128.1.208.* 128.1.209.* 128.1.210.* 128.1.211.* 128.1.212.* 128.1.213.* 128.1.214.* 128.1.215.* 128.1.216.* 128.1.217.* 128.1.218.* 128.1.219.* 128.1.220.* 128.1.221.* 128.1.222.* 128.1.223.* 128.1.224.* 128.1.225.* 128.1.226.* 128.1.227.* 128.1.228.* 128.1.229.* 128.1.230.* 128.1.231.* 128.1.232.* 128.1.233.* 128.1.234.* 128.1.235.* 128.1.236.* 128.1.237.* 128.1.238.* 128.1.239.* 128.1.240.* 128.1.241.* 128.1.242.* 128.1.243.* 128.1.244.* 128.1.245.* 128.1.246.* 128.1.247.* 128.1.248.* 128.1.249.* 128.1.250.* 128.1.251.* 128.1.252.* 128.1.253.* 128.1.254.* 128.1.255.*]
Splitting 128.2-255.*.*
254 subnets: [128.2.*.* 128.3.*.* 128.4.*.* 128.5.*.* 128.6.*.* 128.7.*.* 128.8.*.* 128.9.*.* 128.10.*.* 128.11.*.* 128.12.*.* 128.13.*.* 128.14.*.* 128.15.*.* 128.16.*.* 128.17.*.* 128.18.*.* 128.19.*.* 128.20.*.* 128.21.*.* 128.22.*.* 128.23.*.* 128.24.*.* 128.25.*.* 128.26.*.* 128.27.*.* 128.28.*.* 128.29.*.* 128.30.*.* 128.31.*.* 128.32.*.* 128.33.*.* 128.34.*.* 128.35.*.* 128.36.*.* 128.37.*.* 128.38.*.* 128.39.*.* 128.40.*.* 128.41.*.* 128.42.*.* 128.43.*.* 128.44.*.* 128.45.*.* 128.46.*.* 128.47.*.* 128.48.*.* 128.49.*.* 128.50.*.* 128.51.*.* 128.52.*.* 128.53.*.* 128.54.*.* 128.55.*.* 128.56.*.* 128.57.*.* 128.58.*.* 128.59.*.* 128.60.*.* 128.61.*.* 128.62.*.* 128.63.*.* 128.64.*.* 128.65.*.* 128.66.*.* 128.67.*.* 128.68.*.* 128.69.*.* 128.70.*.* 128.71.*.* 128.72.*.* 128.73.*.* 128.74.*.* 128.75.*.* 128.76.*.* 128.77.*.* 128.78.*.* 128.79.*.* 128.80.*.* 128.81.*.* 128.82.*.* 128.83.*.* 128.84.*.* 128.85.*.* 128.86.*.* 128.87.*.* 128.88.*.* 128.89.*.* 128.90.*.* 128.91.*.* 128.92.*.* 128.93.*.* 128.94.*.* 128.95.*.* 128.96.*.* 128.97.*.* 128.98.*.* 128.99.*.* 128.100.*.* 128.101.*.* 128.102.*.* 128.103.*.* 128.104.*.* 128.105.*.* 128.106.*.* 128.107.*.* 128.108.*.* 128.109.*.* 128.110.*.* 128.111.*.* 128.112.*.* 128.113.*.* 128.114.*.* 128.115.*.* 128.116.*.* 128.117.*.* 128.118.*.* 128.119.*.* 128.120.*.* 128.121.*.* 128.122.*.* 128.123.*.* 128.124.*.* 128.125.*.* 128.126.*.* 128.127.*.* 128.128.*.* 128.129.*.* 128.130.*.* 128.131.*.* 128.132.*.* 128.133.*.* 128.134.*.* 128.135.*.* 128.136.*.* 128.137.*.* 128.138.*.* 128.139.*.* 128.140.*.* 128.141.*.* 128.142.*.* 128.143.*.* 128.144.*.* 128.145.*.* 128.146.*.* 128.147.*.* 128.148.*.* 128.149.*.* 128.150.*.* 128.151.*.* 128.152.*.* 128.153.*.* 128.154.*.* 128.155.*.* 128.156.*.* 128.157.*.* 128.158.*.* 128.159.*.* 128.160.*.* 128.161.*.* 128.162.*.* 128.163.*.* 128.164.*.* 128.165.*.* 128.166.*.* 128.167.*.* 128.168.*.* 128.169.*.* 128.170.*.* 128.171.*.* 128.172.*.* 128.173.*.* 128.174.*.* 128.175.*.* 128.176.*.* 128.177.*.* 128.178.*.* 128.179.*.* 128.180.*.* 128.181.*.* 128.182.*.* 128.183.*.* 128.184.*.* 128.185.*.* 128.186.*.* 128.187.*.* 128.188.*.* 128.189.*.* 128.190.*.* 128.191.*.* 128.192.*.* 128.193.*.* 128.194.*.* 128.195.*.* 128.196.*.* 128.197.*.* 128.198.*.* 128.199.*.* 128.200.*.* 128.201.*.* 128.202.*.* 128.203.*.* 128.204.*.* 128.205.*.* 128.206.*.* 128.207.*.* 128.208.*.* 128.209.*.* 128.210.*.* 128.211.*.* 128.212.*.* 128.213.*.* 128.214.*.* 128.215.*.* 128.216.*.* 128.217.*.* 128.218.*.* 128.219.*.* 128.220.*.* 128.221.*.* 128.222.*.* 128.223.*.* 128.224.*.* 128.225.*.* 128.226.*.* 128.227.*.* 128.228.*.* 128.229.*.* 128.230.*.* 128.231.*.* 128.232.*.* 128.233.*.* 128.234.*.* 128.235.*.* 128.236.*.* 128.237.*.* 128.238.*.* 128.239.*.* 128.240.*.* 128.241.*.* 128.242.*.* 128.243.*.* 128.244.*.* 128.245.*.* 128.246.*.* 128.247.*.* 128.248.*.* 128.249.*.* 128.250.*.* 128.251.*.* 128.252.*.* 128.253.*.* 128.254.*.* 128.255.*.*]
Splitting 129-191.*.*.*
63 subnets: [129.*.*.* 130.*.*.* 131.*.*.* 132.*.*.* 133.*.*.* 134.*.*.* 135.*.*.* 136.*.*.* 137.*.*.* 138.*.*.* 139.*.*.* 140.*.*.* 141.*.*.* 142.*.*.* 143.*.*.* 144.*.*.* 145.*.*.* 146.*.*.* 147.*.*.* 148.*.*.* 149.*.*.* 150.*.*.* 151.*.*.* 152.*.*.* 153.*.*.* 154.*.*.* 155.*.*.* 156.*.*.* 157.*.*.* 158.*.*.* 159.*.*.* 160.*.*.* 161.*.*.* 162.*.*.* 163.*.*.* 164.*.*.* 165.*.*.* 166.*.*.* 167.*.*.* 168.*.*.* 169.*.*.* 170.*.*.* 171.*.*.* 172.*.*.* 173.*.*.* 174.*.*.* 175.*.*.* 176.*.*.* 177.*.*.* 178.*.*.* 179.*.*.* 180.*.*.* 181.*.*.* 182.*.*.* 183.*.*.* 184.*.*.* 185.*.*.* 186.*.*.* 187.*.*.* 188.*.*.* 189.*.*.* 190.*.*.* 191.*.*.*]
Total block count: 827
Merged back to minimal sequential blocks again:
[128.1.0.1-255 128.1.1-255.* 128.2-255.*.* 129-191.*.*.*]
Merged back to range again:
[128.1.0.1 -> 191.255.255.255]