net.IP implements encoding.TextUnmarshaler and encoding.TextMarshaler, so net.IPNet could too.

net.IPNet is a combination of net.IP and net.IPMask. The latter does not implement the encoding interfaces, so maybe IPMask should be changed also.

What should a net.IPMask look like as a JSON string? I see a few options.

• Match IPMask.String - always a hex representation of the mask
• Match IPNet.String after the slash (a decimal integer or hex representation depending on whether the mask is a valid CIDR or not)
• Match IPNet.String, including the slash and following characters (always start with a slash).

Then, independently of the above, should net.IPNet marshal/unmarshal a format matching IPNet.String (as @gucki requested), or just a JSON expansion of the IPNet struct, such as this:

{ "IP": "127.0.0.1", "Mask": "8" }

Too late, I'm afraid. That would change the current encodings.

http://play.golang.org/p/DY7Juqs9vT

package main

import (
    "encoding/json"
    "fmt"
    "log"
    "net"
)

func main() {
    js, err := json.Marshal(net.IPNet{})
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println(string(js))
}

@rsc Sorry, I don't get why parsing "127.0.0.1/24" into a net.IPNET could not be implemented anyway?