Repository: go
Issue: net: IPNet JSON marshal/unmarshal inconsistency
Labels: NeedsInvestigation

<!-- Please answer these questions before submitting your issue. Thanks! -->

### What version of Go are you using (`go version`)?

<pre>
$ go version
go version go1.13.4 darwin/amd64
</pre>

### Does this issue reproduce with the latest release?

Yes

### What operating system and processor architecture are you using (`go env`)?

<details><summary><code>go env</code> Output</summary><br><pre>
$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/elad/Library/Caches/go-build"
GOENV="/Users/elad/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/elad/go"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.13.4/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.13.4/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/12/1cmrxkzd3mjdw85xz7jj36cr0000gn/T/go-build857110316=/tmp/go-build -gno-record-gcc-switches -fno-common"
</pre></details>

### What did you do?

Wrote code that uses `net.ParseCIDR()` to create a `*net.IPNet`, then uses `json.Marshal()` to serialize it to JSON, then uses `json.Unmarshal()` to deserialize it back to `*net.IPNet`.

Here's a link: https://play.golang.org/p/ryc7rp4KyiZ

Code just in case:

```go
package main

import (
	"encoding/json"
	"net"
	"testing"
	
	"github.com/stretchr/testify/assert"
)

func TestMarshal(t *testing.T) {
	var err error

	_, cidrOut, err := net.ParseCIDR("192.168.0.0/16")
	assert.Nil(t, err, "net.ParseCIDR")

	cidrJson, err := json.Marshal(cidrOut)
	assert.Nil(t, err, "json.Marshal")

	var cidrIn *net.IPNet
	err = json.Unmarshal(cidrJson, &cidrIn)
	assert.Nil(t, err, "json.Unmarshal")
	
	assert.Equal(t, cidrOut, cidrIn, "CIDR value differs")
	assert.Equal(t, len(cidrOut.IP), len(cidrIn.IP), "IP length differs")
}
```

### What did you expect to see?

I expected that the original and deserialized values will be equal.

### What did you see instead?

I saw that the original and deserialized values are different.

Specifically, I saw that the original value as returned from `net.ParseCIDR()` uses 4-byte representation and the value as deserialized by `json.Unmarshal()` uses 16-byte representation. The values might be semantically equivalent but the internal byte representation is different. Note that this value (`net.IPNet.IP`) is managed internally and not used at all by the calling code.

I think this might be because `net.ParseCIDR()` masks the `net.IPNet.IP` field with a mask that is limited by the IP address length and by default assumes IPv4, so 4-byte representation. The result is that for an IPv4 CIDR, the internal `net.IPNet.IP` will always be in 4-byte representation. The `IP.UnmarshalText()` function uses `net.ParseIP()`, though, which calls `parseIPv4()`, which in turn uses `net.IPv4()` to construct the `net.IP` that is returned. But `net.IPv4()` uses 16-byte representation ("v4-in-v6 prefix").

I think this is not ideal since in order to get semantics where input is equal to output one has to add code as below, which alters internal data structures:

```go
v4 := cidr.IP.To4(); if v4 != nil {
    cidr.IP = v4
}
```

If this is indeed considered a problem, a naive suggestion would be to add an unmarshal function for `net.IPNet` that uses `net.ParseCIDR()` and thus retaining semantics. I don't know if that's viable though or what unforeseen effects this might have.