It looks correct to me. 192.0.2.1 is the host IP, and /24 is the prefix. What is wrong here ?

Nope, the base IP address of 192.0.2.1/24 is 192.0.2.0, and the String() method use the base, see the test or the source:
https://play.golang.org/p/z05_nDz1EFP

Oh that's right.

@clairerhoda perhaps you might be interested in this issue?

@odeke-em @agnivade if no one is working on it ... can I push the PR?

Up to you and @clairerhoda to figure out. I would suggest to wait for @clairerhoda to have a chance to respond.

Sure. Will wait for @clairerhoda to decide.

@odeke-em @agnivade if @bharaththiruveedula has it done he can open the PR.

sure... then I will send PR

Change https://golang.org/cl/189117 mentions this issue: net: fix the docs in IPNet.String

Sorry, actually this example is correct. I did not analyze @gaissmai's example in detail.

@gaissmai - what your playground example is doing with _, c, _ := net.ParseCIDR("192.0.2.1/24") is that now c is is actually 192.0.2.0/24 and not 192.0.2.1. You can see the godoc for https://golang.org/pkg/net/#ParseCIDR for details.

Please see the following code (https://play.golang.org/p/-1UYrVvjtTs):

package main

import (
	"fmt"
	"net"
)

func main() {
	ip := net.IPNet{IP: net.IPv4(192, 168, 1, 4), Mask: net.CIDRMask(24, 32)}
	fmt.Println(ip.String())
}

And as a further confirmation, (*IPNet).String calls networkNumberAndMask which has

if ip = n.IP.To4(); ip == nil {
	ip = n.IP
	if len(ip) != IPv6len {
		return nil, nil
	}
}

Therefore, I think the docs are correct and there is no issue here.

@agnivade Hi, sorry but I disagree. Your example is a little bit contrived. The type IPNet consists of the netnumber and related CIDR mask. Your example misuse the public fileds of type IPNet to put an IP address in this slot which is not a netnumber for the related CIDR mask..

type IPNet struct {
    IP   IP     // network number
    Mask IPMask // network mask
}

If you use ParseCIDR to generate the IPNet struct you will never get the wrong IP address in this field.

Well yes, but the docs are technically right. Although, 192.0.2.0/24 may be more appropriate for a real world example. Leaving to @mikioh @ianlancetaylor

EDIT: to clarify, I am agreeing with you. Just waiting for a second opinion.