Looking at difference between port listening with/ without loopback address

test_port_listen.go

package main
import (
    "fmt"
    "os"
    "net"
    "log"
    "time"
)
func main() {
    port := os.Args[1]
    withAddr, _ := net.Listen("tcp", fmt.Sprintf("127.0.0.1:%s", port))
    withoutAddr, err := net.Listen("tcp", fmt.Sprintf(":%s", port))
    if err != nil {
    	log.Panic("%v", err)
    }
    fmt.Printf("With address: %v\n", withAddr)
    fmt.Printf("Without address: %v\n", withoutAddr)
    fmt.Println("Sleeping for 100 secs...")
    time.Sleep(100 * time.Second)

    if withAddr != nil {
    	withAddr.Close()
    }
    if withoutAddr != nil {
    	withoutAddr.Close()
    }
}