v0.2.5

ADDED:
* num-addrs subcommand, to get the number of hosts/addresses in a given
  prefix length
* get-net subcommand, to more easily get a single subnet from either the
  beginning or the end of a prefix. (MUCH FASTER than CIDR-splitting!)

TODO

@@ -3,3 +3,5 @@
 - when checking/rendering reserved networks, currently the footnotes aren't returned.
 -- netsplit.IANARegistryFootnote
 -- encapsulated in the IANARegistry.Footnotes
+
+- add new interface, Getter? different formatting?

cmd/subnetter/args.go

@@ -2,11 +2,13 @@ package main
 
 type Args struct {
 	Version         verArgs         `command:"version" alias:"v" description:"Show version information." validate:"omitempty"`
-	SplitCIDR       SplitCIDRArgs   `command:"split-cidr" alias:"sc" description:"Split a network into as many equal subnets of prefix size N as possible." validate:"omitempty"`
+	GetPfx          GetPfxArgs      `command:"get-net" alias:"gn" description:"Get a single subnet from a network at either the beginning or end. (You'll probably want to enable -r/--no-remaining.)" validate:"omitempty"`
+	SplitCIDR       SplitCIDRArgs   `command:"split-cidr" alias:"sc" description:"Split a network into as many equal subnets of prefix size N as possible. (This may have issues/take a very long time if you are splitting a very large network into many very small subnets.)" validate:"omitempty"`
 	SplitHost       SplitHostArgs   `command:"split-hosts" alias:"sh" description:"Split a network into N total number of hosts *per subnet* as cleanly/evenly as possible." validate:"omitempty"`
 	SplitSubnets    SplitSubnetArgs `command:"split-nets" alias:"sn" description:"Split a network into N number of subnets as cleanly as possible." validate:"omitempty"`
 	VLSM            VLSMArgs        `command:"split-vlsm" alias:"sv" alias:"vlsm" description:"Use VLSM (Variable-Length Subnet Masks) to split a network into differently sized subnets." validate:"omitempty"`
 	ExplicitNetwork XNetArgs        `command:"net" alias:"xn" alias:"net" description:"Print information about an explicit network address." validate:"omitempty"`
+	NumAddrs        NAddrArgs       `command:"num-addrs" alias:"na" alias:"addrs" description:"Return the number of addresses/hosts in a given network size. (This saves needing to lookup from the table subcommand.)" validate:"omitempty"`
 	NumNets         NNetArgs        `command:"num-nets" alias:"nn" alias:"nets" description:"Return the number of subnets of a given size that can fit into a given network size. This is MUCH, MUCH FASTER than splitting (if you do not need addressing)." validate:"omitempty"`
 	Parse           ParseArgs       `command:"parse" alias:"p" alias:"read" alias:"convert" description:"Parse/convert output from a previous subnetter run." validate:"omitempty"`
 	Table           TableArgs       `command:"table" alias:"t" alias:"tab" alias:"tbl" description:"Show prefix summaries (by default both IPv4 and IPv6)." validate:"omitempty"`
@@ -49,6 +51,21 @@ type cacheArgs struct {
 	DoResCache bool   `short:"c" long:"cache-reservations" env:"SBNTR_RSVCACHE" description:"Enable caching/cache lookup for reservation data."`
 }
 
+type GetPfxArgs struct {
+	Position string `short:"P" long:"position" choice:"first" choice:"last" default:"first" description:"The position of the subnet within the network." validate:"required,oneof=first last"`
+	Prefix   uint8  `short:"s" long:"size" required:"true" description:"Prefix length/network size in bits (as CIDR number)." validate:"required"`
+	splitArgs
+}
+
+type NAddrArgs struct {
+	Isv6          bool `short:"6" long:"v6" description:"If the prefix given is <=32, specify this flag to indicate that it is an IPv6 prefix and not IPv4. (If it is >32, it is automatically treated as an IPv6 prefix for obvious reasons.)"`
+	InclNetAddr   bool `short:"N" long:"incl-net" description:"If specified, include the network address in the count."`
+	InclBcastAddr bool `short:"B" long:"incl-bcast" description:"If specified, include the broadcast/reserved broadcast address in the count."`
+	Size          struct {
+		PrefixSize uint8 `positional-arg-name:"<prefix length>" required:"1" validate:"lte=128"`
+	} `positional-args:"yes" required:"1" validate:"required"`
+}
+
 type NNetArgs struct {
 	Verbose   bool `short:"v" long:"verbose" description:"Be verbose (more ideal for logging)."`
 	NoV6Check bool `short:"6" long:"no-v6" description:"If specified, do not indicate if the subnetting is IPv6 only (true) or not (false; dual-stack/IPv4 supported)."`

cmd/subnetter/funcs.go

@@ -15,7 +15,6 @@ import (
 	"time"
 
 	"github.com/goccy/go-yaml"
-	"github.com/projectdiscovery/mapcidr"
 	"go4.org/netipx"
 	"r00t2.io/subnetter/netsplit"
 	"r00t2.io/subnetter/version"
@@ -159,8 +158,8 @@ func printMask(label string, pfx netip.Prefix, verb, indent int, indentStr strin
 	fmt.Fprintf(sb, "%sBits:\t\t%d\n", pre2, pfx.Bits())
 	fmt.Fprintf(sb, "%sFirst:\t\t%s\n", pre2, first.String())
 	fmt.Fprintf(sb, "%sLast:\t\t%s\n", pre2, last.String())
-	fmt.Fprintf(sb, "%sAddresses:\t%d\n", pre2, mapcidr.CountIPsInCIDR(true, true, netipx.PrefixIPNet(pfx.Masked())))
+	fmt.Fprintf(sb, "%sAddresses:\t%d\n", pre2, netsplit.NumAddrsPfx(pfx.Masked(), true, true))
-	fmt.Fprintf(sb, "%sHosts:\t\t%d\n", pre2, mapcidr.CountIPsInCIDR(false, false, netipx.PrefixIPNet(pfx.Masked())))
+	fmt.Fprintf(sb, "%sHosts:\t\t%d\n", pre2, netsplit.NumAddrsPfx(pfx.Masked(), false, false))
 	if verb >= 2 {
 		fmt.Fprintf(sb, "%sExpanded:\t%s\n", pre2, netsplit.MaskExpand(mask, pfx.Addr().Is6()))
 		fmt.Fprintf(sb, "%sHex:\t\t0x%s\n", pre2, mask.String())

cmd/subnetter/funcs_tpl.go

@@ -10,7 +10,6 @@ import (
 	`strings`
 
 	`github.com/TwiN/go-color`
-	`github.com/projectdiscovery/mapcidr`
 	`go4.org/netipx`
 	`r00t2.io/subnetter/netsplit`
 )
@@ -423,8 +422,8 @@ func tplTablePrefixes(ipVer uint8, indent string, plain bool) (out string, err e
 			return
 		}
 		dummyNet = netipx.PrefixIPNet(rows[idx].NetPrefix.Masked())
-		rows[idx].Addresses = mapcidr.CountIPsInCIDR(true, true, dummyNet)
+		rows[idx].Addresses = netsplit.NumAddrsNet(dummyNet, true, true)
-		rows[idx].Hosts = mapcidr.CountIPsInCIDR(false, false, dummyNet)
+		rows[idx].Hosts = netsplit.NumAddrsNet(dummyNet, false, false)
 	}
 
 	colFields, colTitles, colSizes = sizeStructs(rows)

cmd/subnetter/main.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"log"
+	`math/big`
 	"net"
 	"net/netip"
 	"os"
@@ -34,6 +35,7 @@ func main() {
 	var buf *bytes.Buffer
 	var res *netsplit.StructuredResults
 	var numNets uint
+	var numAddrs *big.Int
 	var v6Only bool
 	var noStrict bool
 	var strictErr error
@@ -84,6 +86,18 @@ func main() {
 			log.Panicln(err)
 		}
 		return
+	case "num-addrs":
+		if err = validate.Struct(args.NumAddrs); err != nil {
+			log.Panicln(err)
+		}
+		if args.NumAddrs.Size.PrefixSize > 32 {
+			args.NumAddrs.Isv6 = true
+		}
+		if numAddrs, err = netsplit.NumAddrsIn(args.NumAddrs.Size.PrefixSize, args.NumAddrs.Isv6, args.NumAddrs.InclNetAddr, args.NumAddrs.InclBcastAddr); err != nil {
+			log.Panicln(err)
+		}
+		fmt.Println(numAddrs.String())
+		return
 	case "num-nets":
 		if err = validate.Struct(args.NumNets); err != nil {
 			log.Panicln(err)
@@ -195,6 +209,18 @@ func main() {
 			These are all handily-dandily enclosed in a `common` struct type.
 		*/
 		switch parser.Active.Name {
+		case "get-net": // Not a *true* splitter, per se; splitting is required but only for functional reasons.
+			if err = validate.Struct(args.GetPfx); err != nil {
+				log.Panicln(err)
+			}
+			cmnArgs = args.GetPfx.common
+			splitter = &netsplit.PrefixGetter{
+				Pos:          args.GetPfx.Position,
+				PrefixLength: args.GetPfx.Prefix,
+				BaseSplitter: new(netsplit.BaseSplitter),
+			}
+			noStrict = false
+			strictErr = netsplit.ErrBadNumHosts // dummy
 		case "split-hosts":
 			if err = validate.Struct(args.SplitHost); err != nil {
 				log.Panicln(err)

go.mod

@@ -8,7 +8,6 @@ require (
 	github.com/go-resty/resty/v2 v2.16.5
 	github.com/goccy/go-yaml v1.15.23
 	github.com/jessevdk/go-flags v1.6.1
-	github.com/projectdiscovery/mapcidr v1.1.34
 	go4.org/netipx v0.0.0-20231129151722-fdeea329fbba
 	golang.org/x/mod v0.24.0
 	r00t2.io/goutils v1.8.1
@@ -16,18 +15,12 @@ require (
 )
 
 require (
-	github.com/aymerick/douceur v0.2.0 // indirect
 	github.com/djherbis/times v1.6.0 // indirect
 	github.com/gabriel-vasile/mimetype v1.4.8 // indirect
 	github.com/go-playground/locales v0.14.1 // indirect
 	github.com/go-playground/universal-translator v0.18.1 // indirect
-	github.com/gorilla/css v1.0.1 // indirect
 	github.com/leodido/go-urn v1.4.0 // indirect
-	github.com/microcosm-cc/bluemonday v1.0.27 // indirect
+	github.com/stretchr/testify v1.9.0 // indirect
-	github.com/pkg/errors v0.9.1 // indirect
-	github.com/projectdiscovery/blackrock v0.0.1 // indirect
-	github.com/projectdiscovery/utils v0.4.14 // indirect
-	github.com/saintfish/chardet v0.0.0-20230101081208-5e3ef4b5456d // indirect
 	golang.org/x/crypto v0.36.0 // indirect
 	golang.org/x/net v0.37.0 // indirect
 	golang.org/x/sync v0.12.0 // indirect

go.sum

@@ -1,7 +1,5 @@
 github.com/TwiN/go-color v1.4.1 h1:mqG0P/KBgHKVqmtL5ye7K0/Gr4l6hTksPgTgMk3mUzc=
 github.com/TwiN/go-color v1.4.1/go.mod h1:WcPf/jtiW95WBIsEeY1Lc/b8aaWoiqQpu5cf8WFxu+s=
-github.com/aymerick/douceur v0.2.0 h1:Mv+mAeH1Q+n9Fr+oyamOlAkUNPWPlA8PPGR0QAaYuPk=
-github.com/aymerick/douceur v0.2.0/go.mod h1:wlT5vV2O3h55X9m7iVYN0TBM0NH/MmbLnd30/FjWUq4=
 github.com/coreos/go-systemd v0.0.0-20191104093116-d3cd4ed1dbcf/go.mod h1:F5haX7vjVVG0kc13fIWeqUViNPyEJxv/OmvnBo0Yme4=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
@@ -22,58 +20,28 @@ github.com/go-resty/resty/v2 v2.16.5/go.mod h1:hkJtXbA2iKHzJheXYvQ8snQES5ZLGKMwQ
 github.com/goccy/go-yaml v1.15.23 h1:WS0GAX1uNPDLUvLkNU2vXq6oTnsmfVFocjQ/4qA48qo=
 github.com/goccy/go-yaml v1.15.23/go.mod h1:XBurs7gK8ATbW4ZPGKgcbrY1Br56PdM69F7LkFRi1kA=
 github.com/google/uuid v1.3.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
-github.com/gorilla/css v1.0.0 h1:BQqNyPTi50JCFMTw/b67hByjMVXZRwGha6wxVGkeihY=
-github.com/gorilla/css v1.0.0/go.mod h1:Dn721qIggHpt4+EFCcTLTU/vk5ySda2ReITrtgBl60c=
-github.com/gorilla/css v1.0.1 h1:ntNaBIghp6JmvWnxbZKANoLyuXTPZ4cAMlo6RyhlbO8=
-github.com/gorilla/css v1.0.1/go.mod h1:BvnYkspnSzMmwRK+b8/xgNPLiIuNZr6vbZBTPQ2A3b0=
 github.com/jessevdk/go-flags v1.6.1 h1:Cvu5U8UGrLay1rZfv/zP7iLpSHGUZ/Ou68T0iX1bBK4=
 github.com/jessevdk/go-flags v1.6.1/go.mod h1:Mk8T1hIAWpOiJiHa9rJASDK2UGWji0EuPGBnNLMooyc=
 github.com/leodido/go-urn v1.4.0 h1:WT9HwE9SGECu3lg4d/dIA+jxlljEa1/ffXKmRjqdmIQ=
 github.com/leodido/go-urn v1.4.0/go.mod h1:bvxc+MVxLKB4z00jd1z+Dvzr47oO32F/QSNjSBOlFxI=
-github.com/microcosm-cc/bluemonday v1.0.25 h1:4NEwSfiJ+Wva0VxN5B8OwMicaJvD8r9tlJWm9rtloEg=
-github.com/microcosm-cc/bluemonday v1.0.25/go.mod h1:ZIOjCQp1OrzBBPIJmfX4qDYFuhU02nx4bn030ixfHLE=
-github.com/microcosm-cc/bluemonday v1.0.27 h1:MpEUotklkwCSLeH+Qdx1VJgNqLlpY2KXwXFM08ygZfk=
-github.com/microcosm-cc/bluemonday v1.0.27/go.mod h1:jFi9vgW+H7c3V0lb6nR74Ib/DIB5OBs92Dimizgw2cA=
-github.com/pkg/errors v0.9.1 h1:FEBLx1zS214owpjy7qsBeixbURkuhQAwrK5UwLGTwt4=
-github.com/pkg/errors v0.9.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
-github.com/projectdiscovery/blackrock v0.0.1 h1:lHQqhaaEFjgf5WkuItbpeCZv2DUIE45k0VbGJyft6LQ=
-github.com/projectdiscovery/blackrock v0.0.1/go.mod h1:ANUtjDfaVrqB453bzToU+YB4cUbvBRpLvEwoWIwlTss=
-github.com/projectdiscovery/mapcidr v1.1.34 h1:udr83vQ7oz3kEOwlsU6NC6o08leJzSDQtls1wmXN/kM=
-github.com/projectdiscovery/mapcidr v1.1.34/go.mod h1:1+1R6OkKSAKtWDXE9RvxXtXPoajXTYX0eiEdkqlhQqQ=
-github.com/projectdiscovery/utils v0.0.85 h1:JpCVc9GJwJLNHy1MBPmAHJcE6rs7bRv72Trb3u84OHE=
-github.com/projectdiscovery/utils v0.0.85/go.mod h1:ttiPgS2LmLFd+VRBUdgfLKMMdrF98zX7z5W+K71MX40=
-github.com/projectdiscovery/utils v0.4.14 h1:BrEfO4f4P+Hu58jNfjho2aRt/Y4jxKhTVQqs2Ei4670=
-github.com/projectdiscovery/utils v0.4.14/go.mod h1:y5gnpQn802iEWqf0djTRNskJlS62P5eqe1VS1+ah0tk=
-github.com/saintfish/chardet v0.0.0-20230101081208-5e3ef4b5456d h1:hrujxIzL1woJ7AwssoOcM/tq5JjjG2yYOc8odClEiXA=
-github.com/saintfish/chardet v0.0.0-20230101081208-5e3ef4b5456d/go.mod h1:uugorj2VCxiV1x+LzaIdVa9b4S4qGAcH6cbhh4qVxOU=
 github.com/stretchr/testify v1.9.0 h1:HtqpIVDClZ4nwg75+f6Lvsy/wHu+3BoSGCbBAcpTsTg=
 github.com/stretchr/testify v1.9.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
 go4.org/netipx v0.0.0-20231129151722-fdeea329fbba h1:0b9z3AuHCjxk0x/opv64kcgZLBseWJUpBw5I82+2U4M=
 go4.org/netipx v0.0.0-20231129151722-fdeea329fbba/go.mod h1:PLyyIXexvUFg3Owu6p/WfdlivPbZJsZdgWZlrGope/Y=
-golang.org/x/crypto v0.32.0 h1:euUpcYgM8WcP71gNpTqQCn6rC2t6ULUPiOzfWaXVVfc=
-golang.org/x/crypto v0.32.0/go.mod h1:ZnnJkOaASj8g0AjIduWNlq2NRxL0PlBrbKVyZ6V/Ugc=
 golang.org/x/crypto v0.36.0 h1:AnAEvhDddvBdpY+uR+MyHmuZzzNqXSe/GvuDeob5L34=
 golang.org/x/crypto v0.36.0/go.mod h1:Y4J0ReaxCR1IMaabaSMugxJES1EpwhBHhv2bDHklZvc=
 golang.org/x/mod v0.24.0 h1:ZfthKaKaT4NrhGVZHO1/WDTwGES4De8KtWO0SIbNJMU=
 golang.org/x/mod v0.24.0/go.mod h1:IXM97Txy2VM4PJ3gI61r1YEk/gAj6zAHN3AdZt6S9Ww=
-golang.org/x/net v0.34.0 h1:Mb7Mrk043xzHgnRM88suvJFwzVrRfHEHJEl5/71CKw0=
-golang.org/x/net v0.34.0/go.mod h1:di0qlW3YNM5oh6GqDGQr92MyTozJPmybPK4Ev/Gm31k=
 golang.org/x/net v0.37.0 h1:1zLorHbz+LYj7MQlSf1+2tPIIgibq2eL5xkrGk6f+2c=
 golang.org/x/net v0.37.0/go.mod h1:ivrbrMbzFq5J41QOQh0siUuly180yBYtLp+CKbEaFx8=
-golang.org/x/sync v0.10.0 h1:3NQrjDixjgGwUOCaF8w2+VYHv0Ve/vGYSbdkTa98gmQ=
-golang.org/x/sync v0.10.0/go.mod h1:Czt+wKu1gCyEFDUtn0jG5QVvpJ6rzVqr5aXyt9drQfk=
 golang.org/x/sync v0.12.0 h1:MHc5BpPuC30uJk597Ri8TV3CNZcTLu6B6z4lJy+g6Jw=
 golang.org/x/sync v0.12.0/go.mod h1:1dzgHSNfp02xaA81J2MS99Qcpr2w7fw1gpm99rleRqA=
 golang.org/x/sys v0.0.0-20211216021012-1d35b9e2eb4e/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20220615213510-4f61da869c0c/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.29.0 h1:TPYlXGxvx1MGTn2GiZDhnjPA9wZzZeGKHHmKhHYvgaU=
-golang.org/x/sys v0.29.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
 golang.org/x/sys v0.31.0 h1:ioabZlmFYtWhL+TRYpcnNlLwhyxaM9kWTDEmfnprqik=
 golang.org/x/sys v0.31.0/go.mod h1:BJP2sWEmIv4KK5OTEluFJCKSidICx8ciO85XgH3Ak8k=
-golang.org/x/text v0.21.0 h1:zyQAAkrwaneQ066sspRyJaG9VNi/YJ1NfzcGB3hZ/qo=
-golang.org/x/text v0.21.0/go.mod h1:4IBbMaMmOPCJ8SecivzSH54+73PCFmPWxNTLm+vZkEQ=
 golang.org/x/text v0.23.0 h1:D71I7dUrlY+VX0gQShAThNGHFxZ13dGLBHQLVl1mJlY=
 golang.org/x/text v0.23.0/go.mod h1:/BLNzu4aZCJ1+kcD0DNRotWKage4q2rGVAg4o22unh4=
 golang.org/x/time v0.6.0 h1:eTDhh4ZXt5Qf0augr54TN6suAUudPcawVZeIAPU7D4U=

netsplit/conts.go

@@ -5,6 +5,7 @@ import (
 	`net/netip`
 	`sync`
 
+	`github.com/go-playground/validator/v10`
 	`github.com/go-resty/resty/v2`
 )
 
@@ -39,8 +40,12 @@ var (
 )
 
 var (
-	// TODO
+	// validate *validator.Validate = validator.New(validator.WithRequiredStructEnabled(), validator.WithPrivateFieldValidation())
-	cacheLock   sync.RWMutex
+	validate *validator.Validate = validator.New(validator.WithRequiredStructEnabled())
+)
+
+var (
+	cacheLock   sync.RWMutex // TODO
 	cacheClient *resty.Client
 	// IPv4: https://www.iana.org/assignments/iana-ipv4-special-registry/iana-ipv4-special-registry.xhtml#iana-ipv4-special-registry-1
 	// IPv6: https://www.iana.org/assignments/iana-ipv6-special-registry/iana-ipv6-special-registry.xhtml

netsplit/errs.go

@@ -11,4 +11,5 @@ var (
 	ErrBadSplitter  error = errors.New("invalid or unknown splitter when containing")
 	ErrBigPrefix    error = errors.New("prefix length exceeds remaining network space")
 	ErrNoNetSpace   error = errors.New("reached end of network space before splitting finished")
+	ErrUnknownPos   error = errors.New("unknown subnet position")
 )

netsplit/funcs.go

@@ -1,6 +1,7 @@
 package netsplit
 
 import (
+	`bytes`
 	"encoding/json"
 	"encoding/xml"
 	"fmt"
@@ -298,6 +299,113 @@ func Contain(origPfx *netip.Prefix, nets []*netip.Prefix, remaining *netipx.IPSe
 	return
 }
 
+/*
+LastSubnetNet returns the last subnet of prefix length pfxLen from network pfx.
+
+lastSubnet will be nil if pfx is nil or invalid.
+*/
+func LastSubnetNet(pfx *net.IPNet, pfxLen uint8) (lastSubnet *net.IPNet, err error) {
+
+	var nPfx netip.Prefix
+	var ok bool
+
+	if pfx == nil {
+		return
+	}
+	if nPfx, ok = netipx.FromStdIPNet(pfx); !ok {
+		return
+	}
+	if !nPfx.IsValid() {
+		return
+	}
+
+	if nPfx, err = LastSubnetPfx(nPfx, pfxLen); err != nil {
+		return
+	}
+	if !nPfx.IsValid() {
+		return
+	}
+
+	lastSubnet = netipx.PrefixIPNet(nPfx)
+
+	return
+}
+
+// LastSubnetPfx is exactly like LastSubnetNet but using netip.Prefix instead.
+func LastSubnetPfx(pfx netip.Prefix, pfxLen uint8) (lastSubnet netip.Prefix, err error) {
+
+	var ok bool
+	var pfxBytes []byte
+	var bitOffset uint
+	var offset *big.Int
+	var ipInt *big.Int
+	var ipBytes []byte
+	var byteLen int
+	var lastNet *net.IPNet
+	var maxBitLen uint8 = maxBitsv4
+
+	if !pfx.IsValid() {
+		return
+	}
+
+	pfx = pfx.Masked()
+
+	if pfx.Addr().Is6() || pfxLen > maxBitsv4 {
+		maxBitLen = maxBitsv6
+	}
+	if pfxLen > maxBitLen {
+		err = ErrBadPrefixLen
+		return
+	}
+	if pfxLen < uint8(pfx.Bits()) {
+		err = ErrBigPrefix
+		return
+	}
+
+	if pfxBytes, err = pfx.Addr().MarshalBinary(); err != nil {
+		return
+	}
+
+	byteLen = pfx.Addr().BitLen() / 8
+	bitOffset = uint(pfxLen - uint8(pfx.Bits()))
+
+	offset = new(big.Int).Lsh(big.NewInt(1), bitOffset)
+	offset.Sub(offset, big.NewInt(1))
+
+	// Cast the prefix (as represented as bytes) into a *big.Int for some number magic.
+	ipInt = new(big.Int).SetBytes(pfxBytes)
+	// Shift to the first "address" in the prefix/mask it.
+	offset.Lsh(offset, uint(pfx.Addr().BitLen()-int(pfxLen)))
+	// Now add the offset to the base network.
+	ipInt.Add(ipInt, offset)
+
+	// If the base address starts at the "0 address" (e.g. 0.0.0.0 or :: etc....),
+	// this can cause some strange behavior when casting the *big.Int to bytes.
+	// So it gets left-null-padded to the appropriate length for the inet family.
+	ipBytes = ipInt.Bytes()
+	if len(ipBytes) < byteLen {
+		ipBytes = append(
+			bytes.Repeat([]byte{0x00}, byteLen-len(ipBytes)),
+			ipBytes...,
+		)
+	}
+
+	// Create an explicit net.IPNet...
+	lastNet = &net.IPNet{
+		IP:   net.IP(ipBytes),
+		Mask: net.CIDRMask(int(pfxLen), pfx.Addr().BitLen()),
+	}
+	// And then make it a netip.Prefix.
+	if lastSubnet, ok = netipx.FromStdIPNet(lastNet); !ok {
+		return
+	}
+	if !lastSubnet.IsValid() {
+		return
+	}
+
+	return
+}
+
 /*
 MaskExpand expands a net.IPMask's string format.
 Like AddrExpand but for netmasks.
@@ -391,29 +499,29 @@ func MaskInvert(mask net.IPMask) (inverted net.IPMask) {
 	Note that for the single-host prefix (/32 for IPv4, /128 for IPv6), numAddrs will *always* be 1.
 	For point-to-point prefix (IPv4 /31, IPv6 /127), numAddrs will *ALWAYS* be 2.
 */
-func NumAddrsIn(prefixLen uint8, isIpv6, inclNet, inclBcast bool) (numAddrs *big.Int, err error) {
+func NumAddrsIn(pfxLen uint8, isIpv6, inclNet, inclBcast bool) (numAddrs *big.Int, err error) {
 
 	var numBits uint
 	var numRemoved int64
 	var maxBitLen uint8 = maxBitsv4
 
-	if isIpv6 {
+	if isIpv6 || pfxLen > maxBitsv4 {
 		maxBitLen = maxBitsv6
 	}
-	if prefixLen > maxBitLen {
+	if pfxLen > maxBitLen {
 		err = ErrBadPrefixLen
 		return
 	}
-	if prefixLen == maxBitLen {
+	if pfxLen == maxBitLen {
 		numAddrs = big.NewInt(1)
 		return
 	}
-	if (prefixLen + 1) == maxBitLen {
+	if (pfxLen + 1) == maxBitLen {
 		numAddrs = big.NewInt(2)
 		return
 	}
 
-	numBits = uint(maxBitLen - prefixLen)
+	numBits = uint(maxBitLen - pfxLen)
 
 	numAddrs = new(big.Int).Lsh(big.NewInt(1), numBits)
 
@@ -451,6 +559,9 @@ func NumAddrsNet(pfx *net.IPNet, inclNet, inclBcast bool) (numAddrs *big.Int) {
 	if nPfx, ok = netipx.FromStdIPNet(pfx); !ok {
 		return
 	}
+	if !nPfx.IsValid() {
+		return
+	}
 
 	numAddrs = NumAddrsPfx(nPfx, inclNet, inclBcast)
 
@@ -460,21 +571,16 @@ func NumAddrsNet(pfx *net.IPNet, inclNet, inclBcast bool) (numAddrs *big.Int) {
 // NumAddrsPfx is the exact same as NumAddrsNet but for a net/netip.Prefix instead.
 func NumAddrsPfx(pfx netip.Prefix, inclNet, inclBcast bool) (numAddrs *big.Int) {
 
-	var numBits uint
+	var err error
-	var numRemoved int64
 
-	numBits = uint(pfx.Addr().BitLen() - pfx.Bits())
+	if !pfx.IsValid() {
-
+		return
-	numAddrs = new(big.Int).Lsh(big.NewInt(1), numBits)
-
-	if !inclNet {
-		numRemoved++
 	}
-	if !inclBcast {
+
-		numRemoved++
+	// Since we're dealing with existing prefixes/networks, we should never get an error.
-	}
+	if numAddrs, err = NumAddrsIn(uint8(pfx.Bits()), pfx.Addr().Is6(), inclNet, inclBcast); err != nil {
-	if numRemoved > 0 {
+		// But *somehow* in case we do...
-		_ = numAddrs.Sub(numAddrs, big.NewInt(numRemoved))
+		panic(err)
 	}
 
 	return

netsplit/funcs_cidrsplitter.go

@@ -22,6 +22,10 @@ func (c *CIDRSplitter) Split() (nets []*netip.Prefix, remaining *netipx.IPSet, e
 		return
 	}
 
+	if err = validate.Struct(c); err != nil {
+		return
+	}
+
 	if base, ok = netipx.FromStdIPNet(c.network); !ok {
 		err = ErrBadBoundary
 		return
@@ -41,6 +45,15 @@ func (c *CIDRSplitter) Split() (nets []*netip.Prefix, remaining *netipx.IPSet, e
 		return
 	}
 
+	/*
+		TODO: This is *super* slow for tiny subnets in a large net.
+				For future optimzation, first off benchmark to make sure it makes a difference, but
+				chunk out the network (how to find appropriate length?) of n < x < y, where n is the network pfx len,
+				and goroutine each split into a channel.
+				Because this splits *on CIDR boundaries* and we aren't VLSM-ing, remaining never has to be considered-
+				it'll always be clean splitting.
+				See CIDRSplitter.LenSwitch.
+	*/
 	for {
 		if sub, remaining, ok = remaining.RemoveFreePrefix(c.PrefixLength); !ok {
 			if !sub.IsValid() {

netsplit/funcs_hostsplitter.go

@@ -28,6 +28,10 @@ func (h *HostSplitter) Split() (nets []*netip.Prefix, remaining *netipx.IPSet, e
 		return
 	}
 
+	if err = validate.Struct(h); err != nil {
+		return
+	}
+
 	pfx, _ = netipx.FromStdIPNet(h.network)
 	tgt = new(big.Int)
 	tgt.SetUint64(uint64(h.NumberHosts))

netsplit/funcs_prefixgetter.go

@@ -0,0 +1,102 @@
+package netsplit
+
+import (
+	`fmt`
+	`net/netip`
+
+	`go4.org/netipx`
+)
+
+// Split is to conform to a NetSplitter, though a PrefixGetter is *technically* not a splitter.
+func (p *PrefixGetter) Split() (nets []*netip.Prefix, remaining *netipx.IPSet, err error) {
+
+	var ok bool
+	var base netip.Prefix
+	var vlsmS *VLSMSplitter
+	var addr netip.Addr
+	var maxPfxLen uint8 = maxBitsv4
+	var ipsb *netipx.IPSetBuilder = new(netipx.IPSetBuilder)
+
+	if p == nil || p.PrefixLength == 0 || p.BaseSplitter == nil || p.network == nil {
+		return
+	}
+
+	if err = validate.Struct(p); err != nil {
+		return
+	}
+
+	// If the position is "first", we can simply call a VLSM.
+	if p.Pos == "first" {
+		vlsmS = &VLSMSplitter{
+			Ascending:     false,
+			Explicit:      false,
+			PrefixLengths: []uint8{p.PrefixLength},
+			BaseSplitter:  p.BaseSplitter,
+		}
+		if nets, remaining, err = vlsmS.Split(); err != nil {
+			return
+		}
+		return
+	}
+
+	if p.Pos != "last" {
+		err = ErrUnknownPos
+		return
+	}
+
+	// Otherwise this gets... messy.
+	if base, ok = netipx.FromStdIPNet(p.network); !ok {
+		err = ErrBadBoundary
+		return
+	}
+	if !base.IsValid() {
+		err = ErrBadBoundary
+		return
+	}
+	ipsb = new(netipx.IPSetBuilder)
+	ipsb.AddPrefix(base.Masked())
+
+	// First if it's a single host prefix, ezpz gg no re.
+	if base.Addr().Is6() {
+		maxPfxLen = maxBitsv6
+	}
+	if p.PrefixLength == maxPfxLen {
+		nets = make([]*netip.Prefix, 1)
+		nets[0] = new(netip.Prefix)
+		addr = netipx.PrefixLastIP(base)
+		fmt.Println(addr.String())
+		if *nets[0], err = addr.Prefix(int(p.PrefixLength)); err != nil {
+			return
+		}
+		ipsb.Remove(addr)
+		if remaining, err = ipsb.IPSet(); err != nil {
+			return
+		}
+		return
+	}
+
+	// Otherwise this gets... interesting.
+	/*
+		For IPv4, performance NORMALLY would be "fine" on modern hardware with:
+			1. straight CIDR-splitting
+			2. grabbing the last prefix
+			3. condensing the leading prefixes to a new IPSet
+		But even this can take a long time (see CIDRSplitter.Split comments).
+
+		In almost all cases (unless subnetting like, n+12 prefix length),
+		IPv6 takes WAY too long.
+
+		So use the same function (LastSubnetPfx) for both cases.
+	*/
+	nets = make([]*netip.Prefix, 1)
+	nets[0] = new(netip.Prefix)
+	if *nets[0], err = LastSubnetPfx(base, p.PrefixLength); err != nil {
+		return
+	}
+	ipsb.RemovePrefix(*nets[0])
+	if remaining, err = ipsb.IPSet(); err != nil {
+		return
+	}
+
+	return
+}

netsplit/funcs_subnetsplitter.go

@@ -27,6 +27,10 @@ func (s *SubnetSplitter) Split() (nets []*netip.Prefix, remaining *netipx.IPSet,
 		return
 	}
 
+	if err = validate.Struct(s); err != nil {
+		return
+	}
+
 	if base, ok = netipx.FromStdIPNet(s.network); !ok {
 		err = ErrBadBoundary
 		return

netsplit/funcs_vlsmsplitter.go

@@ -21,7 +21,15 @@ func (v *VLSMSplitter) Split() (nets []*netip.Prefix, remaining *netipx.IPSet, e
 	var base netip.Prefix
 	var sub netip.Prefix
 	var subPtr *netip.Prefix
-	var ipsb = new(netipx.IPSetBuilder)
+	var ipsb *netipx.IPSetBuilder = new(netipx.IPSetBuilder)
+
+	if v == nil || v.PrefixLengths == nil || len(v.PrefixLengths) == 0 || v.BaseSplitter == nil || v.network == nil {
+		return
+	}
+
+	if err = validate.Struct(v); err != nil {
+		return
+	}
 
 	if err = ValidateSizes(v.network, v.PrefixLengths...); err != nil {
 		return
@@ -38,10 +46,6 @@ func (v *VLSMSplitter) Split() (nets []*netip.Prefix, remaining *netipx.IPSet, e
 		But, as I expected, netipx ftw again.
 	*/
 
-	if v == nil || v.PrefixLengths == nil || len(v.PrefixLengths) == 0 || v.BaseSplitter == nil || v.network == nil {
-		return
-	}
-
 	if !v.Explicit {
 		sort.SliceStable(
 			v.PrefixLengths,

netsplit/types.go

@@ -31,7 +31,16 @@ type NetSplitter interface {
 
 // BaseSplitter is used to encapsulate the "parent" network to be split.
 type BaseSplitter struct {
-	network *net.IPNet
+	network *net.IPNet `validate:"required"`
+}
+
+// PrefixGetter is a "pseudo-splitter"; it splits according to a given prefix but only returns a specfic subnet (either the first or the last).
+type PrefixGetter struct {
+	// Pos is the position in BaseSplitter.network for the selected PrefixLength.
+	Pos string `json:"pos" xml:"pos,attr" yaml:"Position" validate:"required,oneof=first last"`
+	// PrefixLength specifies the CIDR/prefix length of the subnet.
+	PrefixLength uint8 `json:"prefix" xml:"prefix,attr" yaml:"network Prefix Length" validate:"required,lte=128"`
+	*BaseSplitter
 }
 
 /*
@@ -40,7 +49,11 @@ It attemps to split the network into as many networks of size PrefixLength as cl
 */
 type CIDRSplitter struct {
 	// PrefixLength specifies the CIDR/prefix length of the subnets to split out.
-	PrefixLength  uint8 `json:"prefix" xml:"prefix,attr" yaml:"network Prefix Length"`
+	PrefixLength uint8 `json:"prefix" xml:"prefix,attr" yaml:"network Prefix Length" validate:"required,lte=128"`
+	// TODO: See CIDRSplitter.Split for future optimization using this.
+	// LenSwitch specifies the threshold bit offset after which (inclusive) it switches from a repeated prefix removal to manual recursive binary split.
+	// If 0, 12 is the default.
+	// LenSwitch uint8 `json:"switch_at" xml:"switchAt,attr" yaml:"Switch Offset Threshold"`
 	*BaseSplitter `json:"net" xml:"net,omitempty" yaml:"network,omitempty"`
 }
 
@@ -50,7 +63,7 @@ It attempts to evenly distribute addresses amoungs subnets.
 */
 type HostSplitter struct {
 	// NumberHosts is the number of hosts to be placed in each subnet to split out.
-	NumberHosts uint `json:"hosts" xml:"hosts,attr" yaml:"Number of Hosts Per Subnet"`
+	NumberHosts uint `json:"hosts" xml:"hosts,attr" yaml:"Number of Hosts Per Subnet" validate:"required"`
 	// InclNetAddr, if true, specifies that NumberHosts includes the network address.
 	InclNetAddr bool `json:"net_addr" xml:"netAddr,attr,omitempty" yaml:"Network Address Included,omitempty"`
 	// InclBcastAddr, if true, specifies that NumberHosts includes the broadcast address.
@@ -66,7 +79,7 @@ as cleanly as poossible.
 */
 type SubnetSplitter struct {
 	// NumberSubnets indicates the number of subnets to split the network into.
-	NumberSubnets uint `json:"nets" xml:"nets,attr" yaml:"Number of Target Subnets"`
+	NumberSubnets uint `json:"nets" xml:"nets,attr" yaml:"Number of Target Subnets" validate:"required"`
 	// Strict, if true, will return an error from Split if the network sizes cannot split into equally-sized networks.
 	Strict        bool `json:"strict" xml:"strict,attr,omitempty" yaml:"Strictly Equal Subnet Sizes"`
 	*BaseSplitter `json:"net" xml:"net,omitempty" yaml:"network,omitempty"`
@@ -91,7 +104,7 @@ type VLSMSplitter struct {
 	*/
 	Explicit bool `json:"explicit,omitempty" xml:"explicit,attr,omitempty" yaml:"Explicit Ordering,omitempty"`
 	// PrefixLengths contains the prefix lengths of each subnet to split out from the network.
-	PrefixLengths []uint8 `json:"prefixes" xml:"prefixes>prefix" yaml:"Prefix Lengths"`
+	PrefixLengths []uint8 `json:"prefixes" xml:"prefixes>prefix" yaml:"Prefix Lengths" validate:"required,dive,lte=128"`
 	*BaseSplitter `json:"net" xml:"net,omitempty" yaml:"network,omitempty"`
 }