code: func (p *Pinger) Network(network string) (string, error) {
  	origNet := p.network
  	switch network {
  	case "ip":
  		fallthrough
  	case "udp":
  		p.network = network
  	default:
  		return origNet, errors.New(network + " can't be used as ICMP endpoint")
  	}
  	return origNet, nil
  }
docs: // Network sets a network endpoints for ICMP ping and returns the previous
  // setting. network arg should be "ip" or "udp" string or if others are
  // specified, it returns an error. If this function isn't called, Pinger
  // uses "ip" as default.